Dear Michelle,
I can totally sympathize with the emotions that clearly come through in your letter, but please consider re-casting this letter to emphasize 1) the mythical awesomeness of the teacher and 2) your (also mythical) desire to please the teacher while helping your son to get what he needs. I do not mean to offend in any way, but having dealt with such teacher(s) this year I can tell you that being direct with such people (at least in our experience) and telling them they should be doing more (even if glaringly true) only causes them to dig in their heels and if it comes to that to try to show other faculty/administration how difficult you and/or your child are to deal with (even if all objective evidence is to the contrary). At least in public school you can't get kicked out--Hooray!! But remember your ultimate goal is to get a desirable end result for your kid. If it were me, I might think about reworking it a little to read something similar in spirit to:

Dear Ms. Ferry,
I so appreciate the effort you have made to accommodate DS and particularly the time he spends in his AIG class. You are indeed God's gift to teaching. Unfortunately, I am very concerned because I have been hearing from DS more and more that he does not like school because he does not find it challenging enough--and indeed, he is actually doing the same worksheets that he was doing last year (and still getting them right), so I can understand his frustration. I know that we both want what is best for him, and I know you are so very busy with x kids and would love to do whatever I can to help you--could I perhaps [monitor the lunchroom; bring you an apple every day; bring you coffee] to help the classroom move more smoothly so that you have more time to focus on the kids? I am very concerned that this repetitive material is actually causing DS's lack of focus in school and I very much do not want him to develop a negative attitude towards school and learning. I would be happy to meet with you to discuss any way that I can help.
Your devoted servant,
MI6

I sincerely hope I have not offended in any way. I think it is *ridiculous* (if not downright disgusting) that one might best approach the problem this way, but in our experience even indirectly telling somebody (at least in the South, or at least in NC where we also live!) that they should be doing something just does not get the results you want (even in private school, even when there are two teachers in the classroom. BTW, one of our teachers always finds the time to mention a) how smart her own daughter is and b) how many kids they have in the classroom--but of course she always forgets to divide by 2 because she and the other teacher are co-teachers, not a class where there is a teacher and an 'assistant.') Anyway, please just consider something along these lines, if you feel like you can stomach it (and have the time, etc.). All I can say is that I think being direct, although I agree it absolutely should work, may not. Of course, being indirect may not work either, but I think it has a better shot. Unfortunately I could not do the above with our present teachers with a straight face--although fortunately DD has had teachers where I could have said something similar and absolutely meant it.

Best of luck with this! I very much hope you can find a good solution for you and your DS. And I apologize if anyone was offended--just my opinion, based on our personal experience (that I wish very much had been different).