Washington’s Olney Theatre Center artistic director Jim Petosa plans to leave the theatre at the end of next year to concentrate on running Boston University’s theater school. Since Olney’s new main stage opened in 2005, “it’s been about how do you function now as a fully completed facility,” he says. “Nothing does that better than a search for new artistic leadership. . . . The most potent and creative act I could do on the part of the institution was to allow it to start to ask these questions.”