Being that you are talking about senior year of HS, that opens up many more potential locations, but there are also a few things that you should think about. The French school system in not exactly the same as here in the US. From what my French wife expained to me, they have sort of filtering process that starts between 7th & 10th grade. Some are steered towards trade schools while others to the baccalaureat degree program.
From what she says, they have to allow you to attend school if you live in the area (I am assuming that you and/or your parents have French Citizenship, but this may or may not matter), but it is not clear to me what program you would be able to go into. Might be worth making a short list and calling the local schools to see what might work best. School year in France tends to be kinda long (2 month summer break when my daughter attended ecole maternelle in '07-08), but many places have no school or half days Wednesdays and there are frequent breaks. Some schools may also have some programs that let you get in more skiing, but it might be difficult to step into this your senior year.
Val d'Isere offers great skiing with the (small) town being right at the base of the resort, but it is more of a resort town than a real French town with not much going on out of season. Expect a very small school with a high percentage of transplants, lots of international tourists and english spoken pretty regularly- these may or may not be what you are looking for. In resort at 3 Vallees is likely the same although Moutiers might be more of a year round town. Personally, I think I would pick Chamonix or Bourg St. Maurice (heart of Alps between Espace Killy & 3 Vallees). There is definitely english spoken in Bourg, but you would be forced to use your french there more than in Cham. Cham would be better for moutaineering.
Don't know a lot about it, but Briancon seems like a real mnt town with access to La Grave (and closer access than some of the others to the beaches in the South).
I haven't visited it much but Grenoble would likely be a good option if you prefer a city close to the mnts as opposed to living in a town in the mnts.