Complex sentence: The original site for the school was on Hill Street and Dolly Lane and had a main site and a separate annexe which had maths department, science department and a sixth-form department on the top floor.
Split into simpler sentences: The original site for the school was on Hill Street and Dolly Lane. The school had a main site and a separate annexe, which had maths department, science department and a sixth-form department on the top floor.