Honestly, these AP classes are tough in HS, but I suspect that taking Calc, Physics, and Chem at USNA will be tougher with respect to the material, the speed in which it is covered and the limited time you have to study at USNA. If you cannot do it in HS, how do you think you can be successful at USNA? Also, I am guessing that the admissions folks are looking for these courses as well.
My DS has accepted his appointment for 2017 so my knowledge is minimal and from afar. Maybe some current plebes or youngsters can reply about the benefit, or lack thereof, of taking these AP classes in HS.
Lastly, within reason and given your capabilities and other activities, I would encourage you to push yourself to be the best you can be!
bandad