Im 14 and I feel really weak at school?

Maybe you need to get some rest. I went through that as well, although not as severe. I used to feel so tired and stressed that I couldn't even keep my eyes open in class. Elementary isn't as hard as high school so you're probably just experiencing some changes due to the transition. Try managing your time better and to be able to get your sleep and don't think about your school so much. Grades may be important, but your health is even more important than that. Hope you feel better!