IF ALEXANDRIA decides to establish a city health clinic for youngsters of high school age, what might it reasonably be expected to accomplish? It would provide many kinds of care other than counseling on sexual conduct and providing contraceptives, but that is the part of its work that has generated the current fierce debate. Opponents argue that the clinic would be ineffectual, and that young girls become pregnant not out of any lack of information or access to contraceptives but in response to deep psychological needs. They doubt that the clinic could accomplish much.

Fortunately, there is recent experience in Alexandria on which to form a judgment. For more than a year a small, privately financed clinic, the Fairlington Teen Health Center, has been run by a local health organization in space provided by a church near the city's high school. In the past school year it saw about 1,500 youngsters. They came for many reasons -- some for allergy shots, for example, and some for physical exams required by the school's sports programs. But about two out of five were there for reasons related to sex, and most of those were sexually active girls. A few were pregnant when they first came. But of those who were not, none became pregnant during the year. That is a striking record in a city in which one out of every 10 teen-age girls became pregnant in 1986, and more than half of those pregnancies ended in abortion.

The Fairlington clinic is now open only one day a week, and it's time to turn the job over to the city's Health Department. The supporters of a city clinic to reach those adolescents are well aware of the need to avoid any suggestion that promiscuity is acceptable conduct or that the school is condoning it. While the clinic would be within walking distance of the school, it would be neither in the school itself nor on its grounds.

By way of correction we note that Timothy Elliott, the chairman of the school board, is in favor of the city clinic. He abstained from a vote on it last week, he explains, because a revised resolution had been presented without adequate opportunity for the board to study it.

The decisive vote will be the city council's, probably next month. The new clinic is a promising attempt to deal with a sudden and shocking rise in pregnancies among young girls in a city with the resources to do something about it.