The Mona Lisa Mystery by Pat Hutchins, 1981.
Class 3 from Hampstead Primary School is taking a trip to France with their teacher, Mr. Jones. Miss Barker, their headmistress, was supposed to go with them, but she became ill, so their substitute French teacher, Miss Parker, will be going with them instead. The kids aren’t happy about the substitution because they really like Miss Barker, and Miss Parker isn’t nearly as nice.
From the very beginning of the trip, strange things start happening. A black car follows them to the ferry and even around France. A doctor they meet on the ferry takes a room at their hotel, and unusual characters follow them everywhere they go, some of them in disguise. One of them even enters Jessica and Avril’s hotel room in the middle of the night.
At first, the children think that someone may be trying to kidnap Miss Parker since the mysterious strangers are showing unusual interest in her. Then, the Mona Lisa is stolen while the children visit the Louvre! The thief temporarily holds Jessica hostage before making his escape. Later, the painting shows up at their hotel, and Mr. Jones is arrested as a conspirator in the crime!
The children struggle to unravel the clues and the tangled mass of identities and prove their teacher’s innocence. Who was really following who, who is the real conspirator, and can the children prove it and find the missing painting?