Well, up here at least it's district policy that if you are failing a class you are not allowed to do any sports period, until the next grade check rolls along... So the response really isn't that unwarranted, since I don't remember ever seeing in the constitution the right to participate in sports (could be a good thing to include). But your principle is definitely suffering from a bit of a power trip, and could have handled that in a million better ways.
As far as your french class goes, It's literally all in your head, I understand the problem of not finding motivation to do a class, for college you have prerequisite classes required for all degrees, which forces me to have to take classes like library science, and anthropology... Not exactly very relevant to engineering degrees.
I did the 110 high hurtles, 300 hurtles, and the 4x100, and 4x200 relays in high school, and believe it or not, school work is just like sports, the more you practice at it the better you get.
You can't learn a language by just "doing" the work, just as you cant complete a hurdle race by "only" running, the first hurdle you come up to will just break your kneecaps, and you will face-plant into the track.
The key to learning a language is learning how to think in it, you should look at a pen, and think "un stylo", not look at a pen and go "hmm a pen... whats a pen in french? oh thats right un stylo"
Its not as hard as you might think, for example, when you look at your principle when you see him in the hall you prolly think "asshat" or something along those lines, you don't sit there and think "Hmm Mr.<w/e his name is> what do I call him again? oh that's right jerkface!"
So look you can already do it, and didn't even know it! Amazazing!
Yes I know there is more to french than just memorizing names for things, there are all the male/female word rules, and all that stuff, but if toddlers in France can do it, I'm sure Mr."a book every two days" can do it too if he puts his mind to it. Besides, English is a hundred times more complex than any other language out there, so quit whining and get to work!