child disipline

I have a thirteen year old son he is in grade 6 he stayed back a year when he was ingrade three he then went to grade 4 just passed made it to grade 5 they wanted to keep him back again i helped him in the last term made sure he did school work he just managed to pass now in grade 6 did really bad the first two terms i have since taken him to extra english , third term he did a lot better he then needed to do well in last term, i just got the news that he has failed again. i am at my ends i dont know what to do with him, if i dont sit on his back and make sure he does his work he does not do it. the teacher tells me he does not listen in class always and does not always do his homework this is why he has failed again.  he is to busy been the cool kid at school and not to interested in doing school work. I need to know how i disipline him to get him focused on school and to change his attitude Pleas can you assist me in what to do,    

Thank you
karen