Complex sentence: He came to RGS High Wycombe from Pocklington School where he had been Second Master for a number of years, and before he had taught at Bemrose School, Derby.
Split into simpler sentences: He came to RGS High Wycombe from Pocklington School where he had been Second Master for a number of years. Prior to teaching at Pocklington School, he had taught at Bemrose School, Derby.