D., Anita nailed it -- he may well be bored.
But without more information it's hard for us to advise. And frankly even if we had more information, I'd say, it's time for his parents to get professional help rather than relying on us here online. This is what the school counselor is there for! If he is truly OK at home (and think hard about whether he really is, or if it's just that his school behavior's so bad he seems OK at home by comparison), then the issue is something at school.
His parents need a private meeting with his teacher, the counselor, and anyone else at school who works with him (other teachers he sees a lot, etc.). The boy should not be present. The meeting should be at a time and place when the parents have the teacher/counselor/principal's full attention and a decent amount of time, not a hurried five-minute talk in the lobby with kids all around. Have they ever had such a meeting that focuses on getting some solutions started, not just on reports about bad behavior? You need to work out a clear, step-by-step plan to find out what's bothering him so badly he's acting out and how to deal with it. How does the teacher discipline him and how does he react to it? Will the counselor set up some one-on-one sessions with the boy alone to talk to him and find out what's bugging him? Etc. The parents should come out with an action plan and should stay in touch with the teacher and counselor to ensure they are monitoring their son.
This could be anything so a lot of digging is needed. He could be the victim of a bully that you and his parents know nothing about, and he's acting out to look tough to the bullying crowd. Or he could be academically so bored this is his way of injecting some life into what's a crushingly dull day for him at school. Or he could have learning disabilities and feel he's so far behind in his studies there's no point doing anything now but being a smart-aleck. He could be being influenced by other kids he wants to like him but who are poor role models. He could feel there are problems at home that he's never told you about (has his behavior gotten worse since the new baby came? Jealous of the younger kids? etc.) and his actions at school might be his way of getting his parents' attention (even negative attention is still attention!). There could be a host of issues but his parents, counselor, and teacher have to work carefully with him or he'll clam up and it'll be hard to get the truth out of him. He may not recognize it himself--he's only a kid and they don't always know why they do "bad stuff."
Please don't resort to corporal punishment. He is acting out for a reason; find the reason and deal with that root cause. Firm but meaningful disciplines at home for school behavior are appropriate. Hitting teaches nothing but hitting and it's more meaningful to him to take away the things he really values: his computer or TV time, taking away sporting events he was going to play in or play time with friends or a special trip he was expecting--but always be very clear the taking away is a direct consequence of known behaviors at school. Give warnings of what he'll lose at home if he misbehaves at school, before the taking away begins. Most of all, support and reward him when he IS well-behaved in school--reward the good, rather than hitting for the bad. This means the teacher needs to communicate how he's doing, both good and bad.
Hitting him only teaches him that hitting is an acceptable answer to problems because the people he trusts most show him that it is. If he's not already involved at fights at school, why would his parents want to show him now that hitting is OK? If they hit him, they cannot go back later when he wallops someone and tell him, "Don't hit people." He could legitimately answer, "Well, you hit me. So why is it wrong for me to hit someone else?" Find out causes, work closely with the school staff, reward the good with lots of attention and discipline meaningfully but calmly for the bad, and get the help that's available. If the problems continue, I'd move on to see an outside counselor or child psychologist, but start with the resources you already have at the school.