The Alexandria School Board will consider a proposal tomorrow that would require all high school athletes participating in league sports to maintain at least a "C" grade average. Earlier this year, Prince George's County approved a similar measure.

According to a report released by the Alexandria school administration yesterday, the proposal, considered the stiffest of its kind in the state, could sideline as many as 19 percent of the athletes. Alexandria's proposal is considered tougher because its "C" grade is equivalent to at least a 77; in Prince George's a "C" grade must be at least 70.

Many state and local officials seem increasingly concerned about athletes' academic requirements. The Virginia High School League, the board in Charlottesville that sets the minimum requirements for athletes participating in league sports, will meet with Virginia high school principals Oct. 18 to discuss increasing the requirements. Its recommendation is to change from four to five the number of courses athletes must pass in order to play.

In April, the Prince George's County Board of Education approved a plan to take effect in January 1985 that will exclude all students from athletics and other extracurricular activities if their grades drop below a "C" average. School officials in Fairfax, Arlington and Prince William counties have focused their attention recently on athletes' academic records but are wary of imposing standards that are too harsh.

The Fairfax School Board decided last March not to increase the average required from a passing "D" to a "C," saying that such "a specific level of achievement could unjustly exclude" handicapped students and those with learning disabilities. Like Arlington and Prince William school boards, Fairfax school officials said they would welcome the Virginia High School League's recommendation to increase the number of courses athletes had to pass.

Lynnwood Campbell, the Alexandria School Board member proposing the "C" average measure, said "it only makes sense. We should require that extra effort in academics that coaches always talk about."

Campbell said the number of athletes who maintain less than a "C" average was "totally unacceptable." The report issued yesterday said 152 of the 821 students who participated in the 14 league sports Alexandria offers ended last semester with a "D" average. Campbell said he hoped he could convince the board, which appears divided on the issue, that asking "for satisfactory work is the least we should do."

Many of Alexandria's team coaches plan to lobby hard against the new requirement. Glenn Furman, the coach of the T.C. Williams Titans, the high school football team ranked seventh in the nation, said the proposal "could hurt our program tremendously. We would go from a superior team to a mediocre one."

Furman said that some of the School Board members are "under the false impression that if they athletes are taken away from sports they will spend the time on studies." Not so, Furman said, "the move would just kill student morale. They wouldn't even want to come to school."

A.K. Johnson, the T.C. Williams track coach, agrees with Furman. "God didn't make all of us perfect or even average. Some can make better grades and some can make better athletes."

The School Board will discuss the proposal in a work session tomorrow night, when they will decide whether to put it on the agenda for a vote on Oct. 17.