Where will tomorrow's audiences come from? That's not a question asked at New Jersey's Paper Mill Playhouse. They know. Since 1989 the company has bused in nearly 4,000 high school students for free matinees, sent artists-in-residence to the schools, and given youngsters the opportunity to perform their original plays on the Playhouse stage. The remarkable Adopt-a School-program and its offshoots began nine years ago when the company learned that students at nearby Newark Arts High--one of oldest performing arts high schools in country--weren't going to the theater, mostly because they couldn't afford to.