These are the books by Gillian Cross that are reviewed on this site:
The Demon Headmaster
What lengths will this Headmaster go to to run an orderly school?
Pictures in the Dark
The otter is many things to many people. To Charlie it is a challenge, to photograph in its natural habitat. To Peter it is wild and free, something whichPeter isn't. And to Peter's father it is vermin, and must be exterminated.
Facing the Demon Headmaster
Dinah and SPLAT face their old adversary, the Headmaster.