I agree with everyone else. This is teacher's problem. If this was my child, I'd be trying to convince her to give him work that's more challenging. It's possible she thinks he doesn't understand it when in reality, he may not be paying attention.
You might want to suggest to him that he read the directions so that he can prove to her that he can do the work.
I like MoN's idea to tell her "he says that he works ahead because he has nothing else to do." Then ask her what she wants him to do and tell her to deal with it herself. It's her classroom, not yours.
Do you get annoyed with her when stuff doesn't go the way you want at your workplace? Do you expect her to solve your problems at work? Of course not.
Alternatively, if she's looking for parental involvement in how to run her classroom, perhaps she could consider giving more advanced material to your son so that he doesn't get bored.
