The Prince George's County School Board last night rejected a school desegregation plan proposed by the national NAACP, apparently clearing the way for the NAACP to reopen a federal court suit that was bitterly contested almost a decade ago.

The NAACP had threatened that if the board rejected the plan it would reopen the 1972 suit that resulted in a controversial court-ordered busing plan that is still in effect.

Thomas I. Atkins, NAACP general counsel, said last night he takes "the board's action as an invitation to sue, an invitation we are not likely to ignore."

Last April, the NAACP asserted that the school system, which is 50 percent black, is still segregated and that there is a pattern of discrimination in student discipline and teacher assignments.

Discussions between the law firm of Hogan and Hartson, representing the NAACP, and school board attorney Paul Nussbaum began but were suspended on Wednesday, Nussbaum said, when the NAACP lawyers produced the desegregation plan in the form of a consent decree document.

The decree, which, if agreed to by the school board would be enforced by the federal court, would require the board to bring the proportion of black students in each regular classroom to within 20 percentage points of the black enrollment at each school by next month. The racial composition of all schools would have to be within 20 percentage points of the county's black population percentage as a whole by 1985.

School Board President Jo Ann Bell said the board would continue to "deliberate on this matter," but Nussbaum insisted that he would not advise the board to enter into a consent agreement because such a move would automatically reopen the desegregation case anyway.

School officials expressed both surprise and distress at the NAACP's proposal. "It's asking for almost a total overhaul of the school system," said school spokesman Brian J. Porter. "It's a numbers game pure and simple. They're not talking about people, or personalities, they're talking about numbers."

School officials have blamed any increase in substantially one-race schools on population shifts over the last decade, during which the percentage of black students has risen from 21.3 percent to 50 percent.

Bonnie Johns, the board's only black member, was the sole dissenter in the decision to reject the NAACP proposal.