Well, in our case DS would just get a bad grade and possibly a comment that would say "elaborate" or something like that. Some kids are probably just more tuned in to what the teacher wants/expects and others need more explicit instruction.

At 12, my DS really should advocate for himself and ask the teacher what he should have done differently. But since he doesn't do that (sigh) and just ignores the bad grade, I feel that I need to intervene and explain what was expected.