The Prince George's County Board of Education tightened its student attendance policy last night, cutting in half the number of acceptable unexcused absences before a student fails a course.

The change is another in a gradual series of steps the board has taken to beef up academic standards and crack down on discipline over the last few years.

Under the new policy, students in grades 7 through 12 with five or more unexcused absences in a semester will no longer be allowed to pass a course. If the course is required for graduation, students will be required to make it up.

Students with five or more excused absences must make up the work in order to pass the course. In the past, students could have any number of excused absences.

The policy change comes after recent statements by school officials tying poor academic performance to high absenteeism.

"The message we send . . . is a very clear one," said school board member Thomas Hendershot, who sponsored the measure. "Don't stay out of school unless you absolutely have to . . . . We mean for our youngsters to be in school."

The change brings the county closer in line with other Washington area jurisdictions. Montgomery County, for example, also allows five unexcused absences, Arlington allows four and Fairfax allows three each semester. The District of Columbia has no established policy, school officials said, and in Alexandria, students are allowed 10 unexcused absences per semester.

Sarah Johnson was the only school board member to vote against the measure, arguing that it should be postponed a semester to give students fair warning and provide more assistance to students with absentee problems. "We are not giving students and parents due process," she said. "I do not see how one semester will hurt any of us."

But teachers at the meeting were generally supportive. "Curtailing the amount of absences is fine," said Pat Richards, a social studies teacher at Oxon Hill High School. But she added that the new rule may mean more work for teachers who are expected to notify parents when a student accumulates absences.

"Teachers want to teach kids. If they're not in school, they can't learn," said a school administrator.

The new regulation has been cited by Superintendent John A. Murphy as one effort to address the problem of lower achievement among black students. He recently released data indicating that black students scored significantly lower than their classmates on standardized test scores. At the time, Murphy said he believed the discrepancy could be reduced by lowering absenteeism.

The county has one of the highest absenteeism rates in the state. Figures for the 1983-84 school year showed that attendance in Prince George's, which averaged 90.9 percent, was worse than all other Maryland jurisdictions except Baltimore.

Students also face other stricter requirements. Last year the board approved a rule that students must maintain a C average in order to participate in extracurricular activities. The board also recently stiffened the discipline code to require longer suspensions for students who physically attack a school employe.