Children's book

[Pat Martin]

Can anyone out there help me with the title and author of this : A Children's book about a class of children who are the only class in school on the last day of term. The rest of the school are on a trip but they have had their trip after winning an art prize. Two escaped convicts take the class hostage but are saved by two pupils who have left the classroom earlier, pretending to be ill.
I'll be ever grateful