Hi Nathalie,
I have come up against this problem on numerous occasions (presentations, surveys, checklists...) and the crux of the matter is as follows. The French do not use or understand the British convention, based on our school days, of putting a tick to denote "correct" or "agree" and a cross to denote the opposite. When you ask them to tick a box, they wil invariably put a cross or, if you're lucky, a single diagonal.
Solution: use a different code - get them to put 'O' or 'N' for oui and non. Or give them two boxes for each statement, one to express "agree" and the other for "disagree". Then they can put whatever they like, as long as they leave the other one blank.