Often it's not about whether your right or wrong, but about why you're wrong, what gaps are there in your knowledge, your mental models, your logic, etc. Some of this can be achieved from Q&A type websites, but often they require the questioner to articulate the question quite specifically. They don't let the user do something like say "Here's what I've been working on, what do you think?". And they're not grading essays.
I think there's a need and an opportunity for a web service for learners that goes beyond Q&A type support, but is actually set up with a focus on people submitting homework and having it reviewed. I suspect it might be necessary to pay people for such review given it would be more time consuming that simple Q&A response.
Such a service could incorporate all of the useful features of existing Q&A type services (e.g. reputation scores for reviewers, published reviews that are easily findable by other users, strong code of conduct and moderation, question tagging / categorisation), along with those of paid online micro-tasks (such as those of Amazon Mechanical Turk).