+1 to everything mon just said.
Count your blessings, because there are worse ways for a child to cope with this. My DD continued doing the work, continued keeping well-behaved, and even started conforming to what they thought a child her age should look like, including pretending not to know things she'd been doing for years. But the stress of conforming, and of not having her needs met, resulted in something I can only characterize as psychological trauma: uncontrollable meltdowns over ordinary mistakes (spilt milk... literally!) which escalated in frequency, amplitude, and duration, hitting head against walls, openly expressing a wish to be dead or never born, etc.
This is not a behavior problem, this is a child coping with a bad situation as best he can. The problem is not him, it's the situation. That's where all of the focus needs to be.
If the school is not willing to change the situation, then it'll be up to you. In your place, I'd continue to advocate within the school for the short term, and be ready with Plan B: remove him from school and homeschool him for the rest of the year, and begin a dialog with the middle school to see if they have anything suitable to offer him in 5th grade next year.
When someone tries to play the maturity card, ask them how they would respond in his place. Imagine you were in class, seven hours a day, given the same work he's assigned, subject to the same rules he is, and you had to write out answers explaining why you know 5 x 4 = 20 in full sentences... what would you do?
