This is the height of the high school prom season, so ABC could not have picked a better time to air "One Too Many" (tonight at 8 on Channel 7), a strong and moving drama about one teen-age alcohol statistic.

"One Too Many" was produced as an "ABC Afterschool Special," but was wisely deemed important enough to capture the whole family in the prime-time slot.

It starts slowly, and you have the feeling you've seen it before. But stick with it. In the end, "One Too Many" may pack the biggest wallop of any of the recent crop of morality plays aimed at our young and their problems.

Michelle Pfeiffer, Mare Winningham, Val Kilmer and Lance Guest play the four teen-age protagonists, Annie, Beth, Eric and Tim. They are bright, attractive, affluent teens from caring families. But Eric has an alcohol problem and his heavy drinking has an increasingly dampening effect on the relationships of the young people.

Eric's friends try to figure out how to deal with him, but he insists he has the drinking under control -- and besides, they don't want to alienate him. It is easy for the driving instructor to say, as he does, that "friends don't let friends drive drunk," but it is not so easy to woo an uncooperative and bellicose drunk from behind the wheel of a car -- not even if you're his best friend, not even if you're his girlfriend.

Inevitably the situation will end in tragedy, but a deft and all-too-possible plot twist saves "One Too Many" from being just another preachy dramatization of another teen drunk.

Some statistics, courtesy of ABC, accompany "One Too Many": High school youths are involved in 11 percent of all fatal alcohol-related accidents, although they account for only 5 percent of licensed drivers and for less than 4 percent of total vehicle miles driven. And in 1982, 4,000 teen-agers were killed and 130,000 injured in drunk driving accidents.

For families with teens or soon-to-be teens, there is great value in watching "One Too Many." It's not easy, but if that anguish can be forever contained in the cathode tube, it will be more than worth this tough hour.