What is the name of this Mary Danby book please?

Hi I am looking for one of Mary Danby’s stories I read many years ago about a boy called Tim who lived in the Victorian era at a school. It starts off with ‘when the moon peers in through the window’ and talks about a girl going to an older part of her school and seeing a boy in a red jersey vanishing by a cupboard. Two teachers go to investigate after schoo and find a diary in a hidden cupboard where they almost get trapped. The diary is written by the Victorian headmaster about Tim and how he either froze to death, or couldn't get out the cupboard. Many thanks.