We'd love to be able to review and grade student essays, but, unfortunately, we don't offer this kind of service right now. We're a small team and wouldn't be able to grade all the essays students would submit.
But don't despair! We have lots of resources that can help. First, we have lots of writing advice on our site. Check out our blog for advice on the GRE AWA section. If you feel you need your essays graded by someone (not us), this post will help: