I totally get it! Why? Because I have been in the same boat! My son will be 7 next month and we have had a problem with fits beyond my imagination. I thought I was going to go nuts, I really did. All the while a perfect student at school.
A few weeks back he threw the biggest fit before school, we ended up being late and I made him tell his teacher why. He didn't want to, he begged me not to tell her and he wouldn't say a word so after he went into the classroom I told her and she ended up having a talk with him. I don't know what she said but things have been been better.
I also changed the way I handle the situation. I used to get angry and yell and that would make things worse. Now, I speak in a normal tone and let him know he has one minute to settle himself down. I set the timer and ignore him for the minute. Once the timer goes off I tell him that it is not acceptable to act the way he was acting and if it happens agin I take away the wii. (that's our leverage at this point) he gets no other warnings because I know he understands.
I have notived that when I stay calm through his fits and stick to the consequences, he knows I mean business and he knows not to act out.
I hope that helps a little. A friend of mine said he is dropping off a book for me about raising biys. I'll let you know if it's any good and share the info with you soon:)