The student was 12 years old when he got expelled last spring for having a penknife at school. This fall, as his friends enter eighth grade, he's sitting at home, a statistic in Prince George's County's campaign against school discipline problems.

His mother photocopied every page of his seventh grade books before returning them to his Prince George's County middle school. She tried to buy the eighth grade texts his classmates are using and a gym suit, for him to use if he is eventually accepted back at school. She finds it hard to keep him home studying photocopied pages while she's at work.

"They turned him out in the streets," she says, asking that her son not be identified for fear that it will hurt his chances of getting back into school. "He can go out and commit a crime and walk the streets on bail, but he has a penknife and he gets kicked out. It's just too strong, entirely too harsh for first and second offenders."

The boy was one of 179 students expelled from Prince George's County schools last year. That compares to only a handful in all other Washington jurisdictions combined. No youngsters were kicked out of public schools in the District, Alexandria or Arlington during the 1983-84 school year. Montgomery County schools expelled one student and Fairfax County expelled four.

The Prince George's County Board of Education has mandated expulsion if a student is caught distributing drugs or with a weapon.

"Some may say it's a tough policy, but it should be," says school spokesman Brian J. Porter. "The rule is so well known, a student has to intentionally, flagrantly violate the rule to get caught. We will do what's appropriate and necessary to maintain a safe environment."

An expelled student must stay out of the school district for one full semester before reapplying.

The fate of those expelled is not known because no school officials track the youngsters once they leave the public schools, according to Clark A. Estep, executive assistant to the superintendent. One drug counselor who works with many of these students, however, believes that most do not enroll in other schools.

"If they're old enough, they might work. But most of them sit at home," says Beverly Vayhinger, supervisor of a county drug counseling center in Clinton, where many students expelled for drugs are referred. Most of the students she sees cannot afford private schools, or private schools refuse to accept them because they were expelled, she says.

The 179 students expelled in 1983-84 included 113 for possession of weapons, 62 for possession of alcohol or drugs and four for physical attacks on teachers or other students.

School administrators, principals and teachers generally praise the policy which, when challenged, has been upheld in court. "We can't forfeit the rights of those who want to learn because of the hoodlum," said Superintendent John A. Murphy.

But others, mostly parents and attorneys, are critical. Among their complaints:

*Mandatory expulsion is unfair because it does not allow the superintendent to consider mitigating circumstances. Also, principals are not allowed enough discretion to grant leniency to first offenders or other less culpable offenders. An expulsion can be appealed to the Board of Education which has more discretion, but only one expulsion has been overturned by the board, according to school officials.

*The policy is so strict that some students have been expelled even when criminal charges against them for the same offenses have been thrown out of court for lack of evidence.

*Delays in scheduling hearings and appeals can keep students out of school for weeks before their cases are completed.

*Hearing procedures do not always afford students due process and the student conduct code is not specific enough. The code, for example, does not spell out clearly enough what constitutes a dangerous weapon, one attorney says.

There is also the issue of a public school system's responsibility to all youngsters.

"Our philosophy is that because education is so important, we need to make every effort to try to keep a kid in school," says Kenneth Muir, director of information for the Montgomery County schools. "We look upon expulsion as an absolute last resort. It's sort of an admission of failure in that we have not been able to modify this kid's behavior."

Daniel Brown, director of school-community relations for the Arlington schools says that in that district, "the attitude is to build in as many different kinds of programs as possible. If you expel them, you don't even have the option of helping them."

Drug counselor Vayhinger says she supports a code that outlaws drugs in school. But the students who get expelled are often not the worst offenders, she notes; they are simply less sophisticated and therefore they get caught.

"About 90 percent of the kids I've seen expelled are pretty good kids," she says. "They don't have a history of extensive drug use. They do pretty well in school."

Murphy, superintendent since the beginning of July, said Prince George's policy is lenient compared to student conduct codes he has enforced in other jurisdictions. "There was a drastic reduction in discipline problems," he said.

When the Prince George's policy was adopted, school officials predicted that expulsions would decline as students became more aware of the consequences, but so far that has not happened. Expulsions went from one in 1981-82, to 174 the following year, then to last year's 179.

"It's still a testing process," says Estep. "Once the students understand . . . administrators and the board do mean business, eventually it will decrease."

While both the new superintendent and Board of Education chairwoman Bonnie F. Johns support the tough code, both also have expressed interest in offering more alternatives for students who violate the policy.

"My concern is . . . to offer options outside of school for those kids," says Murphy. "We're not offering that now. When kids are being put out because of our mistakes, then we've got a responsibility to provide alternative situations."

Students who have been expelled from the school system can apply for re-admission only after they have been out one full semester. If expelled for a drug or alcohol violation, they must complete a rehabilitation program before coming back to school.

State law requires the county to provide a free education to all children up to age 16. "We have to allow a child to attend school at least once," says school spokesman Porter. "But if a child violates the rules and procedures, he loses that privilege to attend school. We don't necessarily turn our back on these students. But they have to assume a certain responsibility for themselves."

Most teachers favor the policy, says Paul Pinsky, president of the Prince George's County teachers union.

The question of fairness draws the most complaints.

"A policy that amounts to a mandatory expulsion policy in practice is wrong," says John Roemer, executive director of the Maryland Chapter of the American Civil Liberties Union. "The courts understand that not all offenses are the same. Some people need to get their hand slapped and some need to get kicked out of school. We don't just shoot everybody who does the same thing."