Talk to the teacher and find out what the antecedent is for some of his bad behavior.  In other words, what makes him fly off the handle?  Is it that he doesn't want to share?  Is it that he can't take the word "no?"  If you can figure out what's making him so angry, that will help you prepare for it.

Try pre-teaching.  Find a neutral time, and teach him what you expect from him when he gets angry.  Teach him ways to calm down, like counting to ten or telling the teacher he's mad.  After you teach him, practice with him so he knows what it is he's supposed to do.

Make sure you're giving him positive consequences at home, too, for his good behavior.  Sometimes kids will act out because they want attention, and they don't care what kind it is.

If things continue the way they are, maybe check in with his pediatrician to make sure his behavior isn't indicative of a disorder or anything like that.

Good luck!

MKM, Counselor
Boys Town National Hotline
1-800-448-3000