California State Teachers Retirement System, Sacramento, probably will do an asset allocation study in September, said Patrick Mitchell, CIO of the $98.5 billion fund. The proposal for the study will be made to the board of trustees at its July 15 meeting, he said. The board will then decide if and when to do the study, which probably will occur in September or October. "We wont know anything about our investment plans until after the allocation study. Any changes we make in our investment strategy wont be made until after 2000, Mr. Mitchell said.