I like the good grades idea too. We let our children stay home when they had lots of homework. They did have to do the homework though. We would make sure they didn't watch TV.
My Mom always was sensitive to my homework too. Also sleep deprivation worked on her. I could say I didn't sleep well the night before and just couldn't go to the meeting. I REALLY needed to go to bed right after dinner. She always gave in to that too. I didn't use it very often.