What's this book

[Pat Martin]

A children's story set in a primary school. Only one class is in school on the last day of term as all the others are on a trip. Two escaped prisoners brake into the school and take the class hostage, all except two boys, one of whom has pretended to be sick so that he and his friend can go and mess about in the cloakroom. They eventually rescue the class. I've read this to countless classes over the years. Now retired I want to read it to my grandaughter but can't remember the title or author. I thought it was Anne Fine but it appears not. Thanks in advance for any answers