After Prom, she could do homework
@daughtermitchell I've moved your post to the Sims 3 section.
Has this sim been to school yet as a teen? The old homework, the item from elementary school, won't be usable anymore. It often gets deleted after the first day of high school, but if it doesn't, and you mouse over it, the description should say old homework. The teen should have new homework and be able to do it though.
If this isn't the problem, try dragging the homework out of inventory and onto a desk or table (with a chair available), to see if the teen can do it there. If that doesn't work either, wait one more school day and see whether the homework is reset. The sim can do her homework while at school—one of the options is "work on late homework"—and if she finishes it, that should force the homework to reset at the end of the school day.
By the way, I never have my sims do their homework after they get an A, and as long as they stay in a good mood while at school, their grades don't drop. I do play on normal lifespan though, which may influence the rate of performance gain. So if you can get the teen to do her homework at school and then get herself to an A, the issue might not matter anymore.
Prom shouldn't have anything to do with this issue, nor should the presence of toddlers or pets.
After Prom, she could do homework
