You will do fine
as I would think that any child in a French immersion class is going to be motivated.
It is difficult for others to tell you how to act toward, or handle, 7th graders.
I am a big man with a cold stare and I intimidate 7th graders. I know, because the children of friends and neighbors have told me so, "Mr. H., Darius said you scare him". "Yehh, welll, Darius should be scared".*
If you are five foot two, and pleasing to look at, you will be perceived differently by the children. There is no substitute (no pun intended) for experiance. I doubt you will have much trouble with those kids. Remember, they are seventh graders and let them have a little fun. But if they become a little too out of hand, warn them twice and then kick them out. If you think it might be needed try kicking someone out in the first ten minutes. Make a "statement". But, I would be careful with that practice. Maybe just send them out in the hall for five minutes, then discuss their behavior with them. See if the teachers next door will accomodate one of your toublemakers for ten or fifteen minutes. Man, they hate to open that door and go into another regular teacher's room. I know I would. Be polite. Don't raise your voice. They can outshout you. Document everything. Think before you open your mouth.
*Five years later Darius became a prototypical "Bear Bryant of Alabama" linebacker. 6 foot two, 220 pounds; "Mobile, agile and hostile".