It could be many different things. You could have something medically wrong such as dyslexia which can cause people not to concentrate. You may be a different type of learner then everybody else such as kinetic/visual/audible everyone is different. which may explain why you are okay learning on your own but not in class. you will be developing your own ways of learning. or it could just be that you take a while to get things. i was exactly the same in high school. it was the sciences that got me mostly. i just didnt understand unless the teachers took the effort to help me individually. in the end i taught myself in my own way for me to get it. i revised for my sciences and got a fantastic grades. much to the teachers surprise. If i was you i would have a think as to whether you have trouble reading and writing and concentrating whilst doing so. if this is the case see your school medic or go and see your doctor with your suspicions. obviously read up first so your sure. if not im sure its just the manner in which your learning. if there is anything else i can do to help let me no.

Im going through the same thing, same year/grade and im failing almost all of my lessons. What your explaining is exactly how i feel so i understand what your going through xx