Starting next fall, Alexandria high schoolers will have to produce a C average or better to be eligible for varsity athletics. That's good news.

The new standard-tightening, voted this week by the Alexandria school board, has been roundly endorsed by the city's black leadership. That's better news.

It isn't surprising that the local Urban League and NAACP chapters should favor putting academics over athletics. Everybody does -- in principle. But they also know that the new requirements will work disproportionately against black student-athletes -- perhaps costing some of them a chance at a college scholarship -- and that must have made the decision to endorse the school board action a good deal harder.

Some coaches were disappointed, of course. They cite the danger that T. C. Williams (Alexandria's only public high school) will lose some of its athletes to other schools as a result of the change, or that some youngsters might simply drop out of school. More than 150 athletes ended the last semester with less than a C average.

"We definitely endorse" the higher standard, said Ulysses Calhoun, president of the Alexandria NAACP branch, who saw the coaches' opposition as a desire to continue exploiting young athletes.

The Urban League expressed concern that so many young blacks look to athletics "as a route to success" and often place "little or no emphasis on academic achievement."

Perhaps, though I hope not, some of the ineligible athletes will be disappointed and drop out of school. But there is also a good chance that the move will spare most of them a bigger disappointment down the road.

To begin with, the National Collegiate Athletic Association will shortly tighten standards for college athletes, which means that some of the youngsters who slide through high school without applying themselves to their academic work won't get the college scholarships they are counting on.

But there is a deeper point to the position taken by the school board and the civil-rights groups. Nelson Greene Jr., one of three black members of the school board (all of whom voted for the new rule), complained that coaches "dangle million-dollar contracts" in the faces of impressionable youngsters, leading them to lay undue stress on their athletic development. "No,20,7.3>body tells some of these athletes the chances of becoming a professional athlete."

Those chances are slim indeed. Seven years ago, The Washington Post did the numbers: 700,000 high school boys playing basketball and 1.1 million playing football; only 18,000 basketball players and 43,000 footballers make NCAA teams. Then it gets worse. According to that 1977 report, only 41 rookies made National Basketball Association teams; the National Football League hired only 220 new players.

Thus the odds against a high school athlete's making the pros was something like 5,000 to one in football, and 17,000 to one in basketball. It makes about as much sense as trying to ensure your future by spending your salary on lottery tickets.

There's something else that gifted young athletes never seem to think of. Even that handful of lucky superstars who turn out to be successful professional athletes (and avoid career-ending in2,9.7,1.6>juries) will spend far more of their lives as ex-athletes than as players. What will they do with the rest of their lives if they have negation in favor of sports? These dreamy-eyed youngsters ought to read the excellent piece on ex-Maryland star Len Elmore in today's Sports section. Elmore, who has quit the NBA after 10 years and entered Harvard's Law School, will be all right. He has the guts, the perseverance and the academic background to make it in law school.

If a successful, intelligent pro like Elmore is, at age 32, struggling to get ready for life-after- sports, imagine how much more difficult it will be for the kid who goofs his way through high school and winds up second-string in college.

What young people, even the athletically gifted ones, have to get through their heads is that it makes more sense to prepare for the nonathletic life that is a certainty than to devote all their efforts to preparation for a pro career that is at best a glittery dream. This week's action of the Alexandria school board might help to drive that point home.