Complex sentence: The main house served as the main building of the school and was called Bates Hall, despite that, much of the estate was virtually unchanged, but eventually, other buildings had been built to accommodate the students and teachers needs.
Split into simpler sentences: The main house served as the main building of the school and was called Bates Hall. However, in spite of that, much of the estate was left largely unchanged, but eventually, other buildings had been built on the grounds to accommodate the students and teachers needs.