Help, I Am Being Held Prisoner (V1.1) - Novel 35

An ingrained practical joker, sentenced to prison after one of his jokes got out of hand, is inadvertently dumped into a gang of prison trusties who are able to periodically sneak out of prison to commit robberies for which they have the perfect alibi: they're in jail. The hero is not a criminal by nature, and is terrified but must hide his fear from the gang, who are tough enough that they might kill him if he chickens out. So he uses his skill at practical jokes to surreptitiously disrupt their plans.