POLICE RAIDS on high schools, to enforce the drug laws, are a last resort - but they are an entirely legitimate last resort. In a series of sweeps at high schools throughout Montgomery County, the county police have arrested more than 100 youngsters. Much of the marijuana debate comes down to a matter of opinion, but one point is simple fact: Possession of marijuana is against the law.

Every high school has a responsibility to deliver a variety of important messages to young citizens. One of those messages is that everybody has a right to try to change laws that he considers unjust or unwise. But another is that nobody has a right to pick and choose which laws to obey and which to ignore. To leave children in any doubt at all on that score is profoundly unfair and harmful to them. The police are correct, on educational as well as legal grounds, when they say that enforcement is not to be regarded as optional.

The use of marijuana has long since become widespread in high schools in Montgomery County and most other places. A great many teachers and administrators now accept it as one of life's inevitable nuisances, impossible to eliminate and not worth anyone's trouble to try. Curiously, the same teachers and the same schools take a very different attitude toward alcohol. In all but the most disorganized schools, students understand perfectly well that it is dangerous to bring liquor onto the premises. Even beer can mean trouble. It gets seized, there are suspensions, parents get called.

Why is marijuana treated differently? The reason seems to be historical. It appeared in the schools in the midst of a rebellion - against the Vietnam War, against overheated academic competition - for which a lot of adults had great sympathy. The war is over and the competition for grades is, to judge from the college entrance test scores, considerably relaxed. But the ambivalence about marijuana lingers on.

Under cover police in the schools, it can be argued, change the atmosphere and subtly interfere with the process of teaching and learning. That is not a frivolous point, and it is why this kind of recourse to police intervention is nobody's first choice as a method of enforcement. But marijuana is, after all, more than a symbol of adolescent revolt. It is also a drug. It affects the perceptions of people who smoke it and, like alcohol, it reduces their attention in class to zero. It is the drugs, and not the plainclothes policemen, that constitue the immediate threat to the rate at which these children learn.

The concept of the school as a sanctuary is a deeply attractive one. But it cannot be a sanctuary from the police unless it is also going to be a sanctuary from - rather than for - the drug culture. Incidentally, marijuana is not the only drug involved here; the police report that cocaine has also turned up.

To avoid future police sweeps, Montgomery's school administrators might try enforcing the law a bit more briskly themselves. School Superintendent Charles M. Bernardo already appears to be moving in that direction. But his idea of relying on "peer pressure" - having student leaders exhort the drug crowd - does not seem terribly promising. The responsibility has to lie directly with the adults: principals, administrators, teachers. They are going to have to uphold the prohibitions on drugs with the same sanctions and the same discipline that they uphold the prohibitions on drugs with the same sanctions and the same discipline that they uphold the prohibitions against drinking, thievery, fighting and any other behavior that interferes with orderly at Walt Whitman High School, some of the students threw a public tantrum in the streets, blocking traffic until mid-afternoon - when, of course, it was time to go home. The incident suggested that their education in law and government has yet a long way to go.