Complex sentence: With the addition of the field house, the original gymnasium became obsolete, and starting in the early 1970s much of it was slowly converted into additional classrooms and offices, culminating in 10 additional classrooms and a weight room being added by 1987.
Split into simpler sentences: With the addition of the field house, the original gymnasium had been obsoleted. Because of this, starting in the early 1970s much of the original gym was slowly converted into additional classrooms and offices culminating in 10 additional classrooms and a weight room being added by 1987.