The hard-won $10.2 million school construction bond issue approved by Loudoun County voters in March may not provide enough money to finance all the classrooms the board says it needs to accommodate expected growth, an assistant superintendent for support services told the School Board Tuesday.

Basing his figures on 1987 construction costs, Richard Johnson told the board that if two elementary schools are built as originally planned, there could be a $150,000 shortfall for each project. He said that three fewer classrooms in each school could prevent the deficit. The board's figures were based on 1985 construction costs, a school spokeswoman said.

Johnson suggested that the architect design the schools with the required number of classrooms and that the board not include those classrooms in the base bid. "If the bid comes in low, we can add the classrooms," he said. If it doesn't, Johnson said, any students who would have attended the new schools may have to attend nearby schools instead.

Johnson, whose aggressive management style has been credited for saving Prince William County schools from being overwhelmed by growth during his six-year tenure as superintendent there, said the budget squeeze is caused by higher site development and construction costs and the fact that the board had not included nearly $53,000 for water and sewer hookups for each school.

The bond referendum approved in March was a scaled-down version of a $50.3 million bond issue that voters snubbed last fall. Those who opposed the school construction cited concerns about its costs and the accuracy of the board's growth projections. The bonds will finance construction of an elementary school in the Countryside subdivision in eastern Loudoun where the county's most dramatic growth has occurred, and another in Leesburg, which also is experiencing a building boom.

Johnson, under whose administration Prince William built five schools and doubled its education budget to $161 million, recommended that the board put both school building projects up for bidding in October, when general contractors are planning construction agendas for the coming year. The school system might save money if a single contractor built both schools at the same time, Johnson said.