Five families in Prince William County filed a class action suit in federal court yesterday contesting the county School Board's decision to bus their children up to 20 miles.

The suit, filed in U.S. District Court in Alexandria, is the result of a two-year effort by some families in the county's Montclair section to prevent the board from busing high schools students to balance numerical enrollments in two of the county's six high schools.

In December, the board decided that families in the Montclair area would have to enroll their children in Osbourn Park Senior High, which is located about 18 miles from Montclair, just outside Manassas. The students had been going to Potomac High in their neighborhood.

Potomac enrolled 1,838 students in September, well above its 1,650 capacity. Osbourn, with a capacity for 2,183 students, had an enrollment of 1,578. The busing plan, scheduled to go into effect this Septemeber, would bring enrollment levels to 1,610 at Osbourn and 1,816 at Potomac.

Richard V. Moraski, one of the parents filing the suit, said his 13-year-old daughter would be prevented from participating in after-school activities if she is bused the 20 miles to Osbourn. Bused children won't have time for such activities, he said, and he argued that they will be separated from their classmates at the end of the day--a contention that is disputed by school officials.

"We don't feel Potomac High School is overcrowded. We don't feel sending our kids to Osbourn High School will give them the same opportunity for a total education," Moraski said. "Academics is not the issue. Race is not the issue, It's the total academic opportunity."

Moraski is president of a legal defense fund for the suit that has collected about $12,000.

George Mullen, vice chairman of the School Board, said yesterday that students can participate in after-school activities and catch a special bus that will be provided afterward.

"We have sent children to Osbourn and those parents at first didn't want them to go," Mullen said. "But after they did let them go, parents were very well satisfied. They were satisfied with the school, the quality of education and the fact that we took their children out of an overcrowded school and put them in a school that wasn't overcrowded."

The suit, which alleges that the School Board and the school superintendent denied the families due process, said the board is trying to force residents to support a bond referendum to build another high school by busing children long distances.

Mullen, who had not seen a copy of the suit at the time he was interviewed, said he was not aware of any bond referendum on the board's agenda. He said the board has considered building a seventh high school during the past two years but total enrollment figures were not high enough to warrant the construction.