BEFORE YOU make up your mind about the proposed clinic for high school students in Alexandria, you might want to consider a couple of figures. In 1985, 365 teen-age girls in Alexandria became pregnant. Last year the number soared to 485 -- one out of every 10 teen-age girls in the city, by far the highest rate in Northern Virginia. In 1985, 184 of those pregnancies ended in abortion. Last year 274 ended in abortion. Should the city do nothing about that?

And if you think that perhaps there's a public responsibility to do something, what should it be -- and where? The proposal is to open a city Health Department clinic within walking distance of Alexandria's senior high school, but not in it or on its grounds. The clinic's work would not be limited to sex and reproduction. To the contrary, city officials think that half of its time and effort would be devoted to dealing with drugs and alcohol. On all questions involving sex, the plan is to emphasize counseling on, first of all, personal values. The clinic would dispense birth control information and contraceptives, but as a last resort.

It would be entirely justifiable to object to any kind of program that seemed to give public sanction to promiscuity, or to suggest that it is normal and acceptable conduct. That's what many opponents of the clinic fear. But this proposal is very different. Alexandria's Mayor James P. Moran, who has courageously advocated this project, strongly emphasizes that the clinic will deal with sexual conduct only as one aspect of general health care for youngsters who, not necessarily for economic reasons, are getting less of it than they need. The prototype is a clinic being run by a church near the school, but the costs have curtailed its services. It's time for the city to take over the job.

The School Board voted 6-2 last week to approve the idea in principle. One of the dissenters, Chairman Timothy Elliott, argued that "we need to reinstill moral values in the country so that teens don't have sex and get pregnant." That's an admirable sentiment, and the health clinic would be a valuable ally. Changing moral values is an undertaking of great difficulty in a world in which high school students are surrounded by a popular culture soaked in sexual references. The schools in their excellent sex education courses try to provide another view. The churches are hard at work. But that terrible rise in adolescent pregnancies last year says that much more has to be done. The clinic is the most promising next step. The final decision is to be made, probably next month, by Alexandria's City Council. The clinic deserves its support.