Este é o Groosham Grange de Anthony Horowitz (já confirmado pelo OP nos comentários). Aqui está um resumo que encontrei em uma revisão :
David had been kicked out of a few schools and when he came home and told his parents that he was getting kicked out of his current school, Beton Academy, they were very angry. Suddenly, a letter appears telling them that David had been accepted to a school that promised to whip him into shape. The school, "Groosham Grange" was on it's own island, had only 1 vacation day a year, and parents were never invited to the school to visit!
David gets sent off and on the train and had met 2 friends on the train who were also being sent to Groosham Grange, Jeffrey, a fat little boy who stuttered and Jill, who was rich and rarely saw her parents. [...]
David, Jeffrey, Jill, and all of the other children there are not ordinary children, they are different and special. After they find out what is special about them they have choices to make.