Complex sentence: The College was founded and opened modestly as boys' school on 15 September 1871, and by the end of the first term there were already 21 boys in the school; by the end of the next summer another 32 had joined them.
Split into simpler sentences: The College was founded and opened as a boys' school on 15 September 1871. By the end of the first term there were 21 boys in the school, and by the end of the next summer another 32 had joined them.