I don't actually see a problem with "We must do a lot of homework at this school." It has two interpretations, to me.
First, it could mean "We are required to do a lot of homework at this school", which is something an older student might say to a new student.
Second, it could mean "Apparently we do a lot of homework at this school." It's a little odd, but I can imagine contexts where it would work.
Perhaps they were looking for:
"We do a lot of homework at this school."
I'm sorry I can't be more helpful, moonbeam. The sentence doesn't have a problem, in my opinion.