I agree with the mom who thought changing schools wasnt the answer. Sometimes it may be necessary to change schools but there has got to be a less intrusive and disruptive answer. Have you talked with the school about getting him an IEP (Individual Education Plan) or a 504? These plans can be put into place if the child has a learning disability, an emotional disorder, etc, etc. There are lots of reasons the child can get one. It helps protect the child from disciplinary action if the problem is due to some sort of condition. Have they had him tested for any type of learning disorder? Lots of kids with issues like Dyslexia, ADHD, etc, are very very smart, there is just some sort of obstacle blocking them from success. Also, if he is so smart is it possible he's just not being challenged enough? I would speak with the school about having him tested (it couldnt hurt), even if its not some sort of learning disability at least you know what the problem isnt, that way you can start narrowing down the possibilities.
I would suggest getting him involved with some sort of extracurricular activity but he needs to be performing well academically before he can be trusted with the responsibility of juggling school and other activities. But on the other hand, it may be good thing to get him excited about school, even if its not surrounding academics. It may help him to get his grades up if he wants to play some sort of sport. Usually the school doesnt let kids do extra activities if they are failing. It may be the motivation he needs. I hope some of these ideas help.
I dont know your son or the situation so I suggested some major issues the kids I work with need help with. I hope you find some help in one of the things I threw out there. You know your son best so you'll know which avenue would best suit him.