A federal judge yesterday awarded the Prince William County School Board $2.5 million plus interest from the Ranger Construction Co. of Atlanta in connection with shoddy work and failure to complete the Osbourn Park High School in Manassas.

Judge Oren R. Lewis, sitting in Alexandria, ordered the company to pay costs incurred by the county to complete the school, which opened two years ago, plus courts costs incurred in three years of litigation.

The school board had dismissed Ranger from the $7.4 million school project in 1974 nine months after the school had been scheduled to be finished. At that time, the board contended, the building was still only three-fourth complete and workmanship so poor that roofs leaked, walls fell down and much of the plumbing and interior hardware was inferior or improperly installed.

Ranger had filed suit seeking damages from the school board for its dismissal, claiming the board failed to pay for completed work and failed to live up to an agreement to appoint an expert to settle disputes with Ranger. The company also said the county conducted inadequate inspections of subcontractors work that delayed job progress. That case was dismissed two years ago by a federal jury in Alexandria.