The Prince William Board of County Supervisors issued a stern rebuke last night to the county School Board, rejecting its request to pursue a multimillion-dollar school renovation progam and warning it that untimely spending proposals will not be tolerated.

In a move that left School Superintendent Edward L. Kelly shaking his head, the supervisors turned a cold shoulder to his appeal for permission to apply to the Virginia Public School Authority for $8.4 million in bonds to pay for "urgently needed" improvements and repairs.

Kelly and other school officials had expected little opposition to the proposal, which would have paid for improvements in a failing roof at Woodbridge High School and air conditioning at several schools.

Instead, when Supervisor Edwin C. King (D-Dumfries) moved to adopt the school system's proposal, his motion died in an awkward silence for lack of a second. Some School Board members walked out and later expressed their displeasure at the supervisors' snub.

County Board Chairman Kathleen K. Seefeldt (D-Occoquan) said she did not dispute the merits of the School Board's proposal, only the timing. The request should have been made as part of the county's planning process for capital projects carried out annually in February, Seefeldt said, echoing the comments of other supervisors.

Prince William's 39,100-student school system, the county Park Authority and the county government all have great needs for new facilities, Seefeldt said, and decisions about what projects are most worthy cannot be made piecemeal.

"It's a philosophy of how we are going to govern all the competing agencies and competing priorities," Seefeldt said. "We have to have all of it in front of us at one time."

Other county officials said privately that last night's rejection was intended as a signal that, although school projects are often the most popular items with Prince William residents, the Board of Supervisors, not the School Board, retains control of the county purse strings.

The superintendent, acknowledging he was stung by the turn of events, said he wished the supervisors had given word of their opposition before the request was made, rather than rebuffing him unexpectedly in public.

"Why weren't we told?" Kelly asked, adding that the supervisors' action means that "either the projects aren't funded, or you take away from the instructional budget."