SOLVED. Young Reader fantasy where character can't look back; series, in one of the books he starts on a boat in Marseilles on a school trip and ends up in the catacombs. [s]

[Lobstergirl, au gratin]

Is he French? Is the whole series set in France?
Author's gender?
Covers?
Illustrations?
What makes the book fantasy? Apart from him not being able to look back?

[Ayshe]

World's End maybe?