Your experience with your students and teachers is likely to be much different than mine, so I’ll make my comments short and general. If you have never taught before, and you are expecting to go into your school and change the lives of students who want to learn, you should probably temper your expectations. French students are a lot like American students in many ways (some are intelligent and serious, but most are your typical pre-teens and teenagers), but they are also different in very important ways.
First, French students are much more likely to contest the authority of teachers than the American students I remember...there is a lot of talking back (Mais Monsieur/Madame!), and an almost supreme belief in their own self-righteousness. Second, French students see school as a place where they want to go to socialize and have to put up with lessons while they are there. As a consequence, they talk ceaselessly. Even the French teachers find it impossible to shut their students up all the time, so many just ignore the chatter or come to see it as normal. In short, getting your students to pay attention and disciplining them is probably going to be your most strenuous task, unless you have an exceptional group.
My strategy for dealing with this is to change it up. Seriously...I switch things up. Make them sit in a circle. Give them random partners. Take away their desks and have them sit in facing one another. I feel like there is a constant need to innovate, because your teaching caché as an assistant is novelty. As soon as you lose your novelty, you become like their other teachers (or even worse, like a substitute teacher who can’t control his/her students). My recommendation is that you try to keep that aura of novelty and newness around you as long as you possibly can, but don’t sacrifice your ability to discipline a student who insists on being disruptive.
As far as lesson plans go, it will all depend on what level students you have, and what the teachers want you to do with them. If you have free reign to craft some lessons, they will be very interested in knowing where you come from and what it’s like in the United States. Lessons about schools in the United States went over well, as did lessons that involved visuals, music, and film clips. They love Obama in France, so any way that you can use him to talk about subjects ranging from the complex to the basic will help you.