Casie Dunmore is the top scorer on her high school varsity basketball team and the second best rebounder, but she is a hair's breadth away from getting kicked off the team because she is mystified by congruent triangles.

Sixteen-year-old Dunmore and her winning Bladensburg High School team may be among the first victims of a new Prince George's County rule that prohibits students in the ninth through 12th grades with less than a C average from participating in extracurricular activities. The problem is geometry: if Dunmore doesn't get at least a D in that class when report cards are issued Feb. 5, her grade point average will fall below the minimum 2.0, and she will be watching her teammates from the bleachers.

"If I'm off, I'm going to cry for three weeks," said Dunmore. "I'm trying as hard as I can."

Like other school officials across the country, the Prince George's County Board of Education has beefed up academic standards. It has increased math requirements, for example, and in April, adopted the C-average standard, to go into effect this winter. Alexandria adopted a similar requirement in November, effective next fall.

Most Prince George's school officials agree that the rule has inspired student athletes to spend more time studying. Several coaches have instituted study halls during practice, players are reading textbooks on the team bus and some coaches are requiring players to turn in regular progress reports from teachers.

"If they want to do the sports, they have to do the schoolwork too," said Angelo Castelli, chairman of the Prince George's board and sponsor of the requirement. "Sports is not the end-all."

The rule, however, has potentially devastating effects for some teams. If it had been in effect last year at this time, for example, more than half the winter athletes at Bladensburg High would have been ineligible, according to figures compiled by the schools.

This year, with report cards less than two weeks away, coaches are anxiously awaiting the news.

"I'm nervous. I'm real nervous about seeing those report cards," said Dennis Cope, who coaches the boys' varsity basketball team at Bladensburg. "I'm going to have some who are real close. Running for a championship like I am, we could end up going down the tubes."

Before his game last night, Cope's team was tied for first place in the county under a point system compiled by state athletic officials.

Seventeen-year-old Lemuel Gain-yard is one of those players about whom Cope is worried. Gainyard has received Cs in all his classes except, ironically, physical education, which he is close to failing. The problem, the boy said, is that he is often late or absent from class.

Until this year, students could fail two classes and still participate in extra-curricular activities.

Despite the possibility that they may be off the courts two weeks from now, both Gainyard and Dunmore say they agree with the new rule. And most coaches support the more stringent requirement.

"It might hurt us for maybe a year or so, but in the long run, it will help both the coaches and the athletes," said Ken Lima, athletic director at Suitland High School.

Some coaches have complained that students who improve their grades from a first quarter D to a C in the second quarter will still miss the cut-off because the two grades must be averaged under the regulation.

Dunmore, who earned a 1.5 average in the first quarter, must get a 2.5 average to continue to qualify for the team.

She concedes that the pressure has been an incentive to change her habits. She submitted extra credit reports in English, she said, and postpones marathon phone calls with her boyfriend until her schoolwork is done.

Her teammates, knowing what's at stake, have been helping her with geometry.

"No matter what I do, I can't seem to get that 2.0," she said. "If I'm off, I'm going to work like the devil, because in senior year, I'm definitely not going to be put off."