Complex sentence: Founded in 1947, it was initially controversial because, as a non-denominational school, it wasn't owned by a church but by the Cannon family, who also provided the two headmasters the school had.
Split into simpler sentences: Founded in 1947, it was initially controversial because, as a non-denominational school, it wasn't owned by a church. Instead it was owned by the Cannon family, who also provided the two headmasters the school had.