Schoolwork and homework make me panicky and anxious?

So, I find this very difficult to explain, so bare with me. I panic at the thought of certain classes. Some nights, I lie in bed feeling so anxious I'm shaking and can't sleep, my mind is constantly occupied by thoughts of the school related thing. Also, if I have something I do not understand, I feel my heart racing and I feel physically ill. Thing is, I feel I have no one to confide in, adults just don't seem to remember what it feels like. I can't sit down and do homework without needing a break because I feel panicky. Some days I just don't want to go into school because of particular things. And sometimes I just don't want to do anything and I just want to lie on my bed and, basically, let the panic take over and pass. I once felt panicky at school, in pe, and people could see it in my face I think, I felt dizzy, sick, my breathing was fast, all the classic panic symptoms, and I had no idea what to do. I still don't know what to do. I would really apreciate, (excuse my spelling) some advice on this as I'm seriously stuck. I'm 15, English female, if that helps you. Thanks a lot.