A D.C. school board proposal to require students involved in extracurricular activities to maintain at least a C average would disproportionately affect athletes, according to a study released this week.

Most students involved in extracurricular activities already have grades of C or better and would not be affected by such a requirement, but students involved only in athletics generally have lower grades than students involved in either nonathletic activities or a combination of sports-related and nonathletic programs, according to the study.

The report contains no estimates of the number of student athletes who would be sidelined by the adoption of a C-average rule.

School officials surveyed the grades of 303 students in four city high schools after school board member Eugene Kinlow informally proposed the requirement this spring. Other school board members, parents and teachers have voiced support for the proposal. The board will probably take action on the proposed requirement early next year.

The study "gives opinions from students and parents and coaches and others involved in extracurricular activities," said Superintendent Floretta D. McKenzie. "The purpose is to provide a framework for the board's discussion on the C-average policy presently under review."

In the spring of 1984, the Prince George's County school board became the first such body in the Washington area to require athletes and club members to maintain a C average.

When the rule first took effect earlier this year, 14,195 high school students -- 39 percent of all high school students involved in extracurricular activities -- were declared ineligible.

Some D.C. school board members and officials said they wanted to study the potential impact of such a policy and not rush into it.

"In the four high schools surveyed in the study, students participating in nonathletic programs, such as academic or arts clubs, had a mean grade point average of 2.51 (C plus) and higher attendance rates than student athletes, whose mean grade point average was 1.92 (D). The mean grade point average for all the students was 2.25 (C). "

The high schools included in the survey were Ballou in Southeast, H.D. Woodson in Northeast, and Cardozo and Wilson in Northwest.