The Prince George's County Board of Education voted last night to change its grading policy in an effort to give students incentive to work harder.

Under a policy established two years ago, teachers computing a final course grade were required to give equal weight to the grades a student earned in each quarter of the school year.

Many teachers opposed that system. As John Sisson, executive director of the Prince George's County Educators Association put it: "A kid can make A's in the first two quarters, then not show up for the rest of the year. What kind of system is that? It encourages students to work hard half of the year and then lay back."

Last night the board accepted School Superintendent Edward J. Feeney's recommendation for a modified system giving greater weight to grades earned later in the year in certain cases involving a choice between two grades.

The board also adopted a policy of giving failing grades to students who made E's (the lowest grades) during the third and fourth quartes of a year-long course or during the second quarter of a one-semester course.

Students also will fail a course if they score failing grades in three out of four quarters.

A recent opinion study determined that most secondary school teachers in Prince George's felt that the existing grading system was hurting students' academic performance.