Oklahoma School Land Trust, Oklahoma City, yesterday decided to search for a midcap growth U.S. equity manager to replace George D. Bjurman & Associates, said Herb Johnson, director of investments for the $990 million trust. The new manager will assume Bjurmans $20 million mandate, he said. The decision to terminate was related to performance, Mr. Johnson said. No RFPs were issued. Holbein Associates, the trusts consultant, is handling all proposals. Thomas Berry, director of investments for Bjurman, did not return phone calls by press time. A final decision is expected by May.