Mr Majeika and the School Caretaker by Humphrey Carpenter
When old Mr Jenks retires, St Barty's School advertises for a new caretaker. Unfortunately there's only one applicant - Hamish Bigmore's Uncle Wilf who is just as rude and bad-tempered as Hamish. When Mr Majeika is hurt in an accident it becomes clear that Uncle Wilf is working for the wickedest of witches, Wilhelmina Warlock! It's up to Mr Majeika to work his magic and put things right again.