Leonard Moses, a music teacher at Suitland Senior High School, was rehearsing Bach with a class two years ago when the mellow sounds of the music were shattered by a shotgun blast in the schoolyard. A student with a sawed-off shotgun had critically wounded a classmate.

Now the hallways outside Moses' classroom are quiet, except for the peaceful harmonies flowing from his choral classes.

"Somehow this school's been turned around," said Moses, who began teaching at Suitland High l4 years ago. "The tone of the building is different. Students seem to be better disciplined, more interested in learning and more willing to obey authority."

It is a new era for Suitland High, which like many other Washington-area schools has experienced frequent student disruptions over the last decade. In 1972, students stormed through its corridors in angry protest over court-ordered busing. In 1976, 30 white and black students armed with bottles, rocks and chains battled on the school grounds. And in 1979 came the shotgun incident.

While some behavior problems persist at Suitland and elsewhere, school systems around the Beltway report that they are seeing the results of firmer discipline policies that have gone into effect increasingly since the mid-l970s. In Washington, Maryland and Virginia, while student enrollments have decreased, the number of student suspensions has generally gone up each year -- in some cases dramatically. The new tough attitude is working as a deterrent to marginal troublemakers, forcing them to think twice before misbehaving in school.

In the District of Columbia, for example, "major suspensions," in which students were sent home for between three and 10 days, rose from 600 five years ago, when 125,908 students were enrolled, to 984 last year, when the enrollment had fallen to 99,366. That means students had a five-in-1,000 chance of being suspended five years ago for serious misbehavior; now their chances have doubled.

Similar tough rules now govern student discipline in school systems throughout the Washington area. Students who are late for class, cause classroom disruptions by sleeping, talking or making other noises, or cut classes are disciplined in every school jurisdiction. The punishments range from detention after school to parent-teacher conferences to extra homework and the loss of a student's privilege of participating in a favorite school activity.

Fighting another student or teacher, cheating, vandalism, possession of a firearm, use or possession of drugs or cursing a teacher spells automatic suspension and can lead to permanent expulsion in most school systems.

"I've seen student behavior here get better every year," said Richard W. Johnson, who has been principal of Jeb Stuart High School in Fairfax County for six years. "Students seem to be more aware that getting an education is important. They are more willing to cooperate with authority."

"I think kids have a lot more respect for teachers now than they used to," said William Laigh, president of the 4,000-member student body at Robinson Secondary School, also in Fairfax County. "Students who used to be disruptive or very vocal in class now are more likely to fall asleep in class than to talk."

Kevin Durham, known in his sophomore and junior years at Suitland High as a problem student, seems to have reformed in his senior year, according to his teachers and classmates.

"When I first came to Suitland, I was always fighting or smoking," said Durham, l8, a clean-cut student who was heading for a morning class recently. "I must've been suspended at least 12 times."

But Durham said he decided to improve his behavior after he was told that suspensions this year would kill his chances of graduating next spring and after his mother threatened to send him to an institution if he didn't behave.

Although student enrollments throughout the area have gone down steadily in recent years, tighter discipline has sparked increases in suspensions -- the most frequently used remedy for serious student discipline problems.

Five years ago, the Prince George's County school system, with the area's largest student enrollment of l44,532, had 12,997 suspensions, including some students who were suspended more than once. By the 1980-81 school year, the enrollment had dropped to 121,893. But the number of suspensions had risen to 15,779.

The Prince George's County school system, for a decade the target of discrimination charges by the NAACP, is currently facing renewed NAACP accusations that black students are suspended in a much greater proportion to their numbers in the school system where 75 percent of the teachers are white.

During the last school year, out of a total of 9,591 students suspended countywide, for example, 3,011 were white and 6,430 -- or about 67 percent -- were black students. Yet black students make up only 50 percent of the county's student enrollment, according to school system statistics.

"We can no more easily explain why more black students are suspended in the county than we can why more boys are suspended than girls," said school system spokesman Brian Porter. "We treat all of our students alike and we maintain records for the procedures used to discipline each child."

In Fairfax County, suspensions went from 5,235 in 1976 with a student enrollnment of l34,674 to 8,290 in the fall of 1979 when 128,063 students were enrolled.

And in Montgomery County, suspensions increased from 4,642 in l977, when there were 112,625 students, to 5,409 last year when the total enrollment had declined to 98,843.

"We have implemented more intensive discipline policies in our schools over the last few years," said Harry Pitt, deputy superintendent of Montgomery County Schools. "As a result, we've seen marked changes in our schools. There used to be kids walking the halls and making noise during class time. Now when you walk in, the halls are quiet."

Pitt said there are still numerous discipline problems. "The hard-core troublemakers are still with us," he said. "But generally our students are better behaved than they were a decade ago. We have more parent support and the administration is firmly behind the principals. That wasn't true eight or l0 years ago."

"We feel there has been a lot of improvement in student discipline at our school," said Charles F. Bready, chairman of the task force on discipline and principal of Winston Churchill High School in Potomac. "Nationally, there seems to be a swing toward a more conservative outlook on discipline problems ."

Bready said that teachers, parents and even many students at his school also share the conservative view and support stricter disciplinary practices.

"Without a doubt the discipline situation is vastly improved in our schools," said Andrew E. Jenkins III, deputy superintendent of the D.C. school system. "We have fewer disruptive incidents. We have given increased attention to discipline problems and are receiving more cooperation from parents and the community."

Thomas Harper, principal at Dunbar High School for five years and an employe in the public school system for 19, said he saw major improvements in student behavior during the last decade.

"The mid- and late-'60s was a trying time for the whole society," said Harper, whose school has an enrollment of 1,606 this year. "Students at that time reflected society's moods through protests and a rejection of authority. Now we see a general acceptance of authority and a new respect for the flag. More students seem to be aware that they must prepare for a competitive society."

In Fairfax County, where corporal punishment -- although rarely used -- is still among the recommended forms of punishment for some students, school board members in a resolution last July reaffirmed its support of stricter discipline in county schools.

"The whole idea was not to say anything new about discipline," said board member Mary Collier, who introduced the resolution, "but to renew our statement that the board expects the administration to strive for a high standard of student conduct."

"We've had good discipline in our schools," she added. "As the community becomes more and more serious about achieving a better education for the students, we expect those students to attend classes, be there on time and to do good work."

By all accounts, Suitland High, where outsiders once dominated the halls, drugs flowed freely and fights were numerous, is a different place today.

The key to the new sense of order at Suitland, faculty members say, is Walter Battle, a black man and 26-year veteran of the Prince George's school system, who was assigned to the school four years ago when serious discipline problems among its 75-percent black student body were rampant.

"When Mr. Battle arrived here, we were having a constant problem with outsiders roaming the halls and causing a lot of disruptions," said Robert Poling, a geometry teacher who has worked at Suitland for 13 years. "It has been very important for the teachers to know that if we send a student to the principal's office the student will receive fair, but firm, discipline."

Battle, known widely as a firm disciplinarian, said he is only doing his job.

"I think it's the principal's job to create a pleasant environment in which teachers can teach and students can learn," he said. "We've had a lot of improvements here in the last three years. But we still have many, many discipline problems. At the beginning of the year, my goal is to make sure all of the students know what kind of conduct is expected of them. Then we can all live by the book the county's code of student conduct ."

A daylong survey of student behavior at Suitland revealed that hallways are clear during class periods and that most students address teachers and principals respectfully. But inside the classroom, some teachers said, students still pose regular discipline problems.

Five years ago, Suitland's four assistant principals largely divided their work days between chasing non-students from the school's hallways and disciplining students who had drug-related problems, according to Daly Wolfe, a vice principal at Suitland since the late 1960s.

With quieter hallways, the assistant principals have been able to focus more attention on discipline in the classroom -- the students who are late for class, fall asleep at their desks or talk back to their teacher.

Student conduct violations of this sort can result in a loss of privileges or detention after school. In more serious cases, they can land a student in the Student Discipline Center, a supervised classroom where students being punished report each day to sit in a tiny booth facing a drab green wall and do their class work in silence.

A repeated offender or a student who is guilty of using or possessing drugs, fighting, or cursing a teacher can be suspended or expelled permanently.

Martin A. Coles, a 17-year-old senior who plans to be a professional musician, stood alone during a recent lunch hour at Suitland, watching as other students gathered in small groups in the school's courtyard.

"The kids over there are usually smoking pot," he said, pointing to three girls huddled next to the building. "The guys over there sometimes get up a crap game," he added, with a nod toward several boys gathered around a park bench.

"I use to be a part of that crowd," Coles said. "I was suspended in the 10th grade for fighting a dude who kept bothering me in the class. Now I spend my time thinking more about the future."