ABOUT LONG ISLAND; Heads Up!

They're marching for . . . 

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ON toward Great South Bay marched the La Salle 200. Cannons to the right of them, cannons to the left. Theirs was not to make reply. Theirs was but to do, or face serious demerits.

So into a gale-force wind strode the 200. Rifles in hand, sabers at the ready, the gray line marched, round and round. Theirs was not to reason why. At the La Salle Military Academy everyone marches under dawn skies. From John H. Sununu, the former White House chief of staff, to Anastasio Somoza Debayle, the late Nicaraguan dictator, La Salle alumni have started their mornings with a left, right, left.

The residents of Oakdale start their mornings with the sound of distant drums. When the 200 cadets at La Salle hustle out for formation, the town knows it. Many people who live south of Montauk Highway enjoy walking their dogs to the marching beat of the Drum and Bugle Corps from the academy.

But few actually know what takes place at La Salle, the sole military academy-high school on Long Island. "We're the secret behind the hedges," an associate headmaster, Brother John Guasconi, said. "Military seems such a charged word. Some people think we're a reform school. Others imagine a military base hidden on the South Shore.

"A car pulled up the other day, and a woman got out. She was looking for the preschool," also at the property. "I said: 'It's down the road. This is La Salle.' She suddenly turned white and whispered, 'Where are the missiles?' "

No I.C.B.M.'s lurk behind the tall shrubbery and iron gates that separate La Salle from busy Montauk Highway in Oakdale. This is not a James Bond set or a suburban blackboard jungle.

What La Salle really is, according to its brochure, is "a Catholic college preparatory school, an all-male boarding high school with a J.R.O.T.C. program." It is run by the Brothers of the Christian Schools, known worldwide as the De La Salle brothers, after the founder, John Baptist de La Salle.

The school is on the former estate of Frederick Bourne, who in the late 1800's made his fortune as president of the Singer Sewing Machine and Manufacturing Company. Bourne spared no expense in building the 100-room red-brick mansion that overlooks Great South Bay. La Salle officials bought it in 1927, but the academy's history dates back from 1883, when it was founded in the Bronx.

La Salle, with 205 acres of lawn, woods, lakes and ponds, has a campus as picturesque as any. The few who see it are those who visit the students of the academy or the newer educational buildings: the nonmilitary middle school, grades 7 and 8; the preschool Carriage House school; the International Studies Center, where foreign students learn English and American culture, and Camp La Salle, which is open every summer.

More people used to visit. In the 1980's, bountiful days for education, the La Salle Military Academy had an enrollment of 350. But the failing economy has hit the school hard. Declining enrollment has caused a change of philosophy. To increase interest and demystify the surroundings, the academy is inviting the public to an open house on May 9. A Challenge, a Father Says

The mansion alone is a sight to behold: the ballroom, with its huge skylight and pipe organ; the intricate woodwork in the various sitting rooms, the antique furnishings and the numerous works of art. Even Brother John's office includes two Ming vases.

In a glass-enclosed "morning room" sits Kathleen Goettle of Selden, the admissions assistant and one of the few female faces at La Salle. Next to Ms. Goettle sits a gold cannon. "It's used on special occasions," Brother John said. Outside on the lawns, cannons are as common as Canada geese.

Although the mansion is impressive, it is the students' programs and routines that attracts many parents. Ninety-eight percent of the graduating class last year went to college.

"La Salle challenges my son," said Harry Mariani of Lloyd Harbor, whose family owns Banfi Vintners and whose son John is a junior. "We wanted John to have structure in his daily routine. Cold Spring Harbor High School and the other prep schools we looked at seemed a little too loose in their structure. And that worried us."

John Mariani's life is certainly structured. From his first salute at 7 A.M., until taps at 10:30 P.M. he and the other cadets are on the move. "You have to be somewhere doing something all the time," said a senior cadet, Christian Cruz of Smithtown.

Bells ring at 7 A.M. Then it's off to breakfast, quickly followed by formation and inspection. Shoes have to be polished. So must the non-shooting rifles that the cadets carry for marching or twirling. The La Salle Rifles are the national trick-drill champions among military institutes. 30 Free Minutes

Immediately after formation, classes begin, and they continue until 3 P.M., when there are 30 minutes of "controlled freedom," as Brother John puts it. Then comes an hour or more of physical education, either team sports or individual activities. Back to the residence hall for a shower, and then a march into dinner. From 7 to 7:30 cadets are allowed to watch television, specifically the news.

After the news it's one hour of study, in silence, at the cadet's desk. Another half-hour television break is allowed at 8:30, but most cadets use the time to polish their shoes for the next day's inspection. From 9 to 10, it's another hour of silent study.

Seniors have a slightly less rigid schedule. And as students prove themselves, they are awarded more freedom, "like studying while lying in bed," Brother John said. Although that would be structure enough for many, at La Salle it is in a military environment.

Blowing bugles and marching feet are but a few of the military trappings. Cadets wear uniforms similar to those at West Point. They stand for inspection, salute upperclassmen and earn ranks. "We're not training them to become toy soldiers," Brother John said. "They are learning self-discipline." Yes, This Is the Army

The cadets' quarters resemble barracks, with no personal effects in sight. Beds, minus bedspreads or comforters, are neatly made, and black lockers contain uniform parts arranged just so, even down to the smiling socks. (Fold a pair of socks together and see if you can make the fold resemble a smile. La Salle students have it down to a science.)

For this is the Army, Mr. Jones, albeit a teen-age version. In fact, Army officers tour the school every April as part of the accreditation process. "They go over this place top to bottom," Brother John said. "Since the 1950's, we have been an honor unit with distinction."

The strict adherence to military procedure gives La Salle the right to appoint three students to each United States military academy. But few students use the offer. "If one or two a year go, we're doing well," Brother John said. "The cadets say after three or four years here, they've had enough of training as soldiers."

Stephen Thorley of Deer Park is the sole appointee this year. Stephen, the battalion commander, is off to West Point. "I always liked the Army," he said.

Surprisingly, many students say they do not mind the military structure. "It's what makes us responsible," Cadet Cruz said.

"It's what makes La Salle," added a freshman, Alexander Watson of Montauk. His two older brothers went to La Salle. But not every student can tolerate the restrictions.

"This is a tough place for kids," said the Commandant, Brother Peter Iorlano. "They have the permissive society out there, and then they come here. It takes a lot of self-discipline to make it." Opposition to Female Students

Last summer a study was conducted on changing some restrictions and dropping the word military from the school's name. "The alumni had a real problem with that," Brother John said. "They felt the tradition of La Salle would suffer."

Alumni and parents were also opposed to the other subject of the study, the possible addition of girls. "There was very strong resistance to that," Brother John said. "Single-sex education has proven to have more positive results."

There are females around, anyway. "We have some women instructors and we have dances for the cadets," Brother John said.

Still, sexual segregation takes some getting used to. "By Wednesday you start thinking, 'I'm not going to see any girls until Friday,' " Cadet Watson said.

But a senior, Ron Palumbo of Manhasset, said: "It doesn't bother me anymore that they're not here. I go home every weekend and see my girlfriend. And not having girls here means fewer distractions."

Although La Salle is a boarding school, 80 percent of the students go home on weekends. That is because 75 percent come from New York State and 70 percent of those students are from Nassau and Suffolk Counties and Queens and Brooklyn. The five-day boarders pay $12,150 for tuition, room and board. Uniforms are extra.

Seven-day boarders pay more. "That's the other 25 percent," Brother John said. "We have 10 percent national and 15 international. The majority of the foreign students come from Latin America. But we have some from the Pacific Rim, too."

"Academicwise," the principal, Brother Thomas Casey, said, "they run the gamut."

The reasons the young men choose La Salle are as diverse. Some attend because of the educational excellence. Others come for the structure.

"My father received a brochure when I was in sixth grade," Cadet Palumbo recalled. "He said, 'If you ever step out of line, this is where you're going to go.' " And there he was. "I was having too much fun in public school," the young man said. Some Notable Alumni

When he graduates this spring he will join notable alumni like Mr. Sununu; Judge Peter F. Cohalan, the former Suffolk County Executive; a Hollywood producer, John M. Frankenheimer; Assemblyman Charles O'Shea of Bellmore; Peter O'Malley, president of the Los Angeles Dodgers, and Michael G. O'Neil, chairman of the General Tire and Rubber Company. And then there is General Somoza, the dictator, whose name and picture appear nowhere in La Salle literature.

Most foreign students spend their weekends at the school. Washing laundry becomes the highlight, said David Kwok, a sophomore from Venezuela. "It's the only time we have fun," he said. "Otherwise we're shining shoes. We're studying."

The seven-dayers are driven to a local laundry, where they learn to sort colors and choose hot or cold. "We're teaching the democratization of the spirit," Brother John said.

With parents' permission, a cadet can leave the grounds on Saturdays and Sundays. "We take the L.I.R.R. to Massapequa and go to the mall," said Peter Lee, who lives in Norway, where his father is a NATO commander. Freedom is still limited. Underclassmen have to be back at the school by 11 P.M. Seniors may stay out until midnight.

The academy sponsors trips to New York City, and there is an annual ski trip to Vermont. But most weekends Oakdale residents see the cadets at a local shopping center, eating a deli hero or renting "Terminator II" at a video store. Camaraderie 'Something Special'

It is not all work at La Salle. Senior-class pranks have included painting the cannons pink and taking a car up the steps into St. Joseph's Hall. The auto was left in front of the commandant's office.

"The camaraderie here is something special," Cadet Palumbo said. The most important lesson he has learned at La Salle, he said, "is the meaning of friendship, life and purpose."