The Alexandria School Board voted last night to tighten academic requirements for the city's high school athletes, a controversial move denounced by athletic coaches but applauded by black leaders.

A spokesman for the coaches said they fear the requirements may persuade some athletes to attend schools elsewhere.

But the NAACP, the Washington Urban League and all three black members of the nine-member board supported the requirements, under which athletes, beginning next fall, must earn a C average either the semester before the playing season or during it if they want to continue to play league sports.

"We definitely endorse it," said Ulysses Calhoun, president of the NAACP's Alexandria branch. "Coaches and schools are exploiting athletes," said Calhoun, adding that a great number of football and basketball players are black.

While the Alexandria School Board says it has not determined how many of the 152 athletes who ended last semester with less than a C average are black, Deborah Anderson, assistant director of the Washington Urban League, said, "I suspect that a bigger number of black athletes will be affected initially. But in the long run it will be better for all students."

In a written statement to the School Board, the Urban League said that after a national conference focusing attention on "black athletes, on and off the playing field, it was apparent that a number of our young people looked to the playing field as a route to success and very often placed little or no emphasis on academic achievement."

"Nobody tells some of these athletes the chances of becoming a professional athlete. They just dangle million-dollar contracts in front of them," said Nelson Greene Jr., one of the black school board members who voted for the new rule.

Don Riviere, athletic director at T.C. Williams High, the city's only high school, led about a dozen coaches in opposing the stiffer requirements. He said after the vote: "There is a danger of losing some athletes." Throughout the past two months coaches have said some athletes would be forced to drop out of school or move to another school system if the new rule was adopted.

The new requirements approved by the Alexandria board are similar to ones passed by the Prince George's County Board of Education in April and reflect a growing national concern about athletes' academic records.

The National Collegiate Athletic Association recently toughened its eligibility requirements for incoming freshmen. After surveying the records of 16,000 freshman athletes, NCAA legislative aide Kathleen Hatke said, "they found that only 18 percent of the black athletes met the new SAT Scholastic Aptitude Test and curriculum requirements."

Alexandria School Superintendent Robert Peebles said he understands why black groups supported the new requirement: "I think across the country we have become careless and have neglected athletes academically." Especially for many black youths, he said, sports has been touted as the "quick route to success and yet only an extremely small number of people make it."

Peebles supported the measure, as did several other board members on the condition that in exceptional cases, such as when an athlete is working up to his academic potential and still earning less than average grades, the school principal may allow him to play.

The board's vote was 7 to 2. Board member Tim Elliott voted against the measure as did Judy Seltz, saying such a proposal "risked punishing students" before a thorough study had been done.