It won't mess up your head physically I think, but when you're 14, there are many things that 6you still don't know and don't understand, and smoking "quite a lot" of pot will make it harder for you to learn things and register information, even moreso than an older person. Yes it can in fact hamper things which are very important at your age. If I were you I'd only smoke at non-important times. I'd never go to school stoned if I were you. I'd also cut back maybe. It's not fun being a burnout at 15.