Complex sentence: Around 1970, the school added the 3 story building with at least 30 classrooms and they added a cafeteria on the bottom floor with lockers in the hallway that have never been used since the 2004 addition.
Split into simpler sentences: Around 1970, the school added the 3-story building with at least 30 classrooms. A cafeteria was added on the bottom floor with lockers in the hallway that have not been used since the 2004 addition.