In 1972, another referendum was passed adding Oak Park and River Forest, Riverside Brookfield, and Ridgewood high school districts to the original three, forming Community College District 504. This district was expanded to its current size in 1974 by the addition of Rosemont and Pennoyer school districts. By 1975, enrollment had grown to nearly 20,000, in-district tuition had increased to US$11 per semester hour, and Triton had become the largest single-campus community college in Illinois. The campus was expanded by the acquisition of the North Avenue Drive-In theater, which closed in 1973. This area, designated the "East Campus" and located across Fifth Avenue from the original ("West") campus, was cleared and used mainly for the construction of athletic facilities, as well as a small auditorium as part of the Collins Center.