EXPELLING a student from high school is a last resort, and no school system does it without regret. But it's a great deal better to expel a few students than to allow an atmosphere of contempt for the rules to take over a school. Prince George's County's secondary schools have thrown out 174 youngsters, under the new code that went into effect a year ago, for having brought weapons, alcohol or drugs into the schools.

School officials express some surprise that the rate of expulsions did not decline over the year as word of the new penalty spread. But the school board member who proposed the penalty, Angelo Castelli, observes that the number of cases is not extraordinarily high for a system with more than 58,000 secondary school students. The county is wrestling with kinds of misbehavior that are deeply ingrained in this country.

There's still a tradition, diminished but not yet gone, that regards drunkenness as comic. And the fascination with weapons is not, unfortunately, limited to high school kids. The weapon most commonly carried into the schools is a knife. The pocket knife carries a homespun connotation of Huckleberry Finn and that sort of thing. But in a fight that might otherwise have ended in nothing worse than a bloody nose, it can do fatal damage. There is a place in this world for pocket knives--but that place isn't school.

Here and there schools have demonstrated it is possible to uphold the idea that they are a sanctuary from which pills, bottles and knives are excluded--with, if necessary, the people who persist in bringing them. But it isn't done easily or quickly.

For a good many years, American high schools have accepted a responsibility to try to hold every last student in the classroom until graduation. That's a position that does a school credit, but on the whole it's been overdone. In the past decade, things reached a point at which drop-out and explusion rates became a matter of reproach to the school, not to the youngsters who departed or to their families. That was unfortunate because it left a good many thoughtless young people with the impression that it was somebody else's job to get them through school--that they had a right to diplomas regardless of anything they might do or refuse to do.

A modest but useful readjustment of responsibilities is now in progress in Prince George's and in most American school systems. Life is full of necessary rules, and it is a valuable contribution to everybody's education to make it clear that, even in high school, they have to be enforced.