Complex sentence: Thirty teachers were employed in September 1973, the last of whom retired in 2007, at that time there were 25 support staff, including 8 cleaners and 9 kitchen assistants.
Split into simpler sentences: Thirty teachers were employed in September 1973, the last of whom retired in 2007. At that time there were 25 support staff, including 8 cleaners and 9 kitchen assistants.