I'm still getting letters from an April column in which I questioned the advisability of school-based birth control clinics. Many of the writers seem convinced that I am either (1) a mean-minded old fogey who sees pregnancy as the appropriate punishment for the sin of sex or (2) a hopeless idiot who believes that the best way to keep young people moral is to keep them ignorant.

"Face it," one of them wrote. "The reality is that teen-agers are going to engage in sex no matter what their parents say. The most we can do is to try to shield them from the consequences, and that includes birth control and sex education."

That comes close to being the rationale for school-based birth control clinics. It is a rationale I find disturbing, both for old-fogey reasons of morality but also for reasons of practicality. Let me try once more to explain.

The moral side is quickly stated: those charged with the education and development of our children have a responsibility not to abdicate fundamental values, even when they are widely ignored. That "everybody is doing it" is, in the first place, not true and, in the second, no justification for abandoning our duty to say to the young people under our charge: "You shouldn't."

As to pragmatics, the danger is one of unintended (though perhaps predictable) consequences.

In the case of adolescent sex, the reasoning goes something like this: since some children are going to have sex, no matter what we say, the thing to say is not "don't" but "be careful."

If it were only the insistently sexually active who got the "be careful" message, it would do no harm and possibly a great deal of good. The danger comes from saying "be careful" to those adolescents who, with a bit of encouragement, would be willing to postpone sexual activity. And the mixed message -- "Don't, but if you do here's how to avoid the consequences" -- is hardly better.

The result is to undercut our advice -- to imply that we don't really mean what we say -- and, thereby, to weaken the defenses of our children against the pressure of their peers.

This undercutting happens in many ways. president of the University of Virginia, where drug abuse has been a problem, spoke to his newly appointed anti-drug task force. "I would not accept as our goal a 'tolerable' level of drug or alcohol abuse," he said.

But to set a goal as zero drug abuse, said one member of the task force, would be to send the wrong message. The correct one? "To set some limits" while conveying that "we are concerned about caring" for people.

Which strikes me as so much nonsense. It may be true that, given the incidence of drug abuse in the general society, creating a drug-free campus is an impossible dream for a public university the size of Virginia. But it ought to be the ideal. And there is no doubt in my mind that for responsible officials to talk only about setting "some limits" is to undercut the ideal, and quite possibly to increase the number of drug abusers.

I would also suggest that for the university to set up an on-campus drug treatment facility is to communicate the notion that some drug abuse is perfectly acceptable. That is the nature of mixed messages.

The role of responsible adults, it seems to me, is to communicate the unacceptability of behavior we believe to be detrimental. That doesn't mean that I am opposed either to drug treatment facilities or birth control clinics. It means only that to have them on campus, under the sponsorship of those whose job is to say "don't," is to send a mixed message.

What is needed is a way to help the few who are weak, impressionable or incorrigible without seducing the many who might otherwise pay us heed.