Trying To Find A Children's Book

[triciamartin]

I'm trying to find a book which tells the story of a class of children left in school when the rest of the school is on a trip. Two escaped prisoners enter the school and take the children hostage. Two naughty boys, one called Lenny, foil the plot. I thought it was written by either Anne Fine or Penelope Lively but it seems I was wrong.
Any suggestions greatly appreciated,
Pat Martin