More than 38 percent of the high school students in Prince George's County are ineligible to participate in extracurricular activities this semester because they did not earn a C average during the fall semester.

The percentage of ineligible students decreased slightly, from 43.9 percent in the last reporting period at the end of the 1985 spring semester. When the policy first went into effect after the 1984 fall semester, 39.2 percent were ineligible.

The current 38.1 percent translates into 13,833 of the 36,307 students in ninth to 12th grades. They are excluded from sports, clubs and other activities under a policy approved by the county Board of Education two years ago.

At that time, Prince George's became the first jurisdiction in the area to adopt a grade requirement. The concept has become more common across the country, has been adopted by Alexandria, and is under consideration in the District.

In Prince George's, school officials have reported that about 8 percent of those declared ineligible were participating in activities.

"We're not satisfied with 38 percent of our children scoring below a C," said Prince George's school spokesman Brian J. Porter, "and would hope the reforms under way now would improve that."

He said the county is implementing a system of testing designed to carefully measure student progress. Also, school officials have revamped their organizational structure and adopted an "effective schools" approach that emphasizes leadership and a concentration on learning.