I did a modified version of this. After talking with his teacher about how his math wasn't really at his level, I emailed and asked her if I could substitute out his homework for something more challenging. She didn't respond. So I just started doing it three days later. I got a note back about a week later that she would get a different math book for him to do for homework.
I think the key is that it wasn't a surprise to her or an insult to something she was doing. It really only affected my time and energy since it was homework.
