The D.C. Board of Education voted last night to force high school students to balance athletics with academics by requiring participants in sports and other extracurricular activities to maintain a C average, or else remain on the sidelines for nine weeks.

The unanimous vote followed only 20 minutes of discussion among the 11 board members, all in support of the policy. The requirement takes effect this fall and will be based on the final nine-week grading period of the school year just ended. Summer school grades may be used to earn eligibility.

The policy, which is similar to requirements set recently in several other area jurisdictions, applies to all students in grades 9 through 12 with the exception of those who will be seniors in September.

School Superintendent Floretta D. McKenzie told board members last night: "We recognize this is not an easy policy, but a very complex one. We do, however, plan to implement the policy with vigor."

"In the urban environment," McKenzie continued, "there are a lot of things not academic that encourage young people to stay in school. We will also evaluate and share with you any problems which arise from this policy."

Even the board's student representative spoke in favor of the requirement. Monica Jones, a senior at Coolidge High School this fall who is on the school track and weight-lifting teams, told board members the new policy "will encourage me and my fellow students to study harder and to stand up as athletes. It's academics first, extracurriculars second."

There are about 16,000 students in the city's 13 high schools, including about 1,200 involved in athletics and about 3,000 who take part in other activities such as marching bands and foreign language clubs.

The C or 2.0 grade-point average rule applies to all extracurricular activities except student government. However, officials say it is the student athletes who are most likely to fall afoul of the new policy. According to a D.C. school study last year, athletes tend to fall below the average in both grades and attendance.

Athletics is "the most visible of the extracurricular activities, and it's the athletes who are at the greatest risk," said David Huie, director of quality assurance for the school system.

Huie said a survey of 300 students from four high schools found that students who participated in athletics alone had a grade-point average of 1.92, students who participated in sports and other extracurricular activities had a 2.14 average, and those who chose nonathletic activities had average grades of 2.35.

When the Prince George's County school board instituted a C-average policy last year, about 40 percent of students involved in extracurricular activities were declared ineligible. Hoping to avoid such an initial shock, D.C. school officials two months ago allocated $100,000 to pay teachers for special tutorial classes in English, math, science and social studies.

A similar booster program smoothed the way for Alexandria high school athletes, who were faced with the C-average standard this year. After school officials offered tutorial help to students in academic trouble, only 3 percent of athletes failed to make a C average.

The idea of linking academic and athletic performance has been widely debated in recent years. Critics of the "C-or-sit" policy have suggested it could be particularly hard on some black students who might consider sports an alternative career path.

In Fairfax County, for instance, the School Board decided not to impose a minimum academic standard because opponents argued that many students with lower scholastic standards vested a special pride in their extracurricular activities. Students in after-school activities who have failing marks are encouraged, but not required, to seek help.

In Texas, where Gov. Mark W. White Jr. pushed through the first statewide "no pass-no play" rule, the 10,000-member Texas High School Coaches Association voted unanimously to form a political action committee to oppose White's reelection. The group also filed a lawsuit to invalidate the rule. Under the stringent Texas rule, a single failing grade forces a six-week suspension from extracurricular activities.

On the other hand, the National Collegiate Athletic Association this year imposed its own minimum average rule, requiring players to earn a C average in high school and a combined score of 700 (out of a possible 1,600) on the Scholastic Aptitude Test to qualify to receive an athletic scholarship or to play in college.

In other action, the board voted 8 to 3 to require teachers to report Aug. 27, three days before the Labor Day holiday. The vote followed a heated 1 1/2-hour debate in which supporters of the proposal were heckled by teachers in the audience. Last year, when the earlier schedule was first imposed, some teachers protested by staying home.

The board also voted to establish bonuses for teachers who accept traditionally hard-to-fill positions and to increase salaries for all public school teachers by 2 percent.