Complex sentence: The colleges' main building was built in 1935, when it was a separate girls and boys secondary modern schools, it contained many classrooms and the boys schools hall, library and offices and the girls school hall, library and offices.
Split into simpler sentences: The colleges'main building was built in 1935, when it was a separate girls and boys secondary modern schools. It contained many classrooms and the boys' school hall, library and offices and the girls' school hall, library and offices.