Complex sentence: He then went to a council school which he hated but provided him with the prototype for Mr Short in the Avenue Story; this was followed by a grammar school whose dedicated teachers formed the basis of several of his characters.
Split into simpler sentences: He then went to a council school, which he hated but which provided him with the prototype for Mr. Short in "The Avenue". This experience was followed by a grammar school whose dedicated teachers inspired several of his characters.