Les mômes

The good ones know more grammar than you do but are, under any circumstances, loathe to prove it. So don't speak French to them and let them struggle. Many reasons for this but the most important one I can think of is that they will make a natural effort to communicate with you in English that they won't make if they know you speak French. It's easy--even the senior students bought the act. (Speak French to your colleagues behind closed doors).

Relax. Nothing serious ever happened on my tenure. I never even came close to sending a student out of my class. French students are playful. They'll test you, but don't react. Laugh it off instead. If they know you speak French I think they will test you harder, as it takes a real meanness to be shitty to a foreigner--such meanness is usually corrected by peers. Ask attentive students to ask the inattentive ones to be still. This works better than if you do it yourself. Yelling at them in Spanish or Serbo-Croat also has a one-time surprise effect.