In high school I put to much time into football and parties and not enough time into learning harder classes.
Your biggest obstacle in the Nuc Pipeline won't be your lack of Physics (as many other have said before me).
Your biggest obstacle will be the sentence I quoted above. The classwork will be long and extensive. You should start out expecting 12hr days at school, plus several hours at school each and every weekend. [It probably won't be that bad the whole way through school, but you're smarter if you start out expecting it to be that way.]
You will have to keep your nose in the books for serious amounts of time, to the detriment of just about everything else. You won't have to give up
all of your gym/sports/party time, but you will have to give up a whole lot of it. If you can do that, then you probably have a better than 90% chance of succeeding.
Regarding Boot Camp, there's more to it than just Physical Training. Boot Camp is an indoctrination, and you will probably spend more time working on folding your clothes "the right way" than you will running and jumping and doing pushups. Some people definitely do struggle with the physical part of Boot Camp, but it's mainly about learning to follow orders (even mundane ones) and working together with complete strangers as a unit. For example, if it's anything like it was when I went through, you will spend MUCH time marching in formation.
Good luck, and thank you for your future service.
- Greg