WASHINGTON (AP) _ Parents and educators don't seem to mind high school students working more than 20 hours a week after school, despite evidence it interferes with their grades, researchers say.

Parents, teachers and counselors are ''unwilling or unable to convey to adolescents who want to go to college the existence of a safe 'upper bound' in the number of hours (worked) while attending high school,'' acording to a report from the George Washington University Graduate Institute for Policy Education and Research.

Adults should ''start applying the brake,'' the study said.

The study was carried out by George Washington professors Philip W. Wirtz and Cynthia A. Rohrbeck and researchers Ivan Charner and Bryna Shore Fraser of the National Institute for Work and Learning.

Previous studies have indicated that although part-time work can have a positive impact on youths' performance in high school, their grades start to trail off when they clock more than 20 hours a week on the job.

The researchers questioned 446 college-bound students who worked in seven fast-food chains. The 58 percent who worked at least 20 hours a week had a B average, while those who worked less than 20 hours averaged B-plus.

Most students said parents and school officials approved of their work, no matter how many hours it entailed.

''Part-time employment while in high school has a number of appealing features. ... (It) can instill proper work attitudes and habits,'' the study said. But ''it may be time for concern about too much of a good thing.''

More than 2.4 million high school students hold part-time jobs during the school year, including nearly a third of ninth and tenth graders and three- quarters of seniors, according to the researchers. One in six flips hamburgers and performs other chores in fast-food restaurants.