Range-of-motion problems are fairly straight forward doing the squat. If you can't do full-squats then you should probably not do squats at all.
Smith-machines or whatever the hell your gym teacher has is shit. Find a real squat rack. Your max might go down 40-20 pounds, but 300-320 isn't bad for someone your weight anyways.
The only suggestion I will give you, and this perhaps will be true with the majority of people on this forum is to find a squat rack. That probably means find a different place to workout, which may or may not be a pain in the ass.