I work for a company that specializes in this exact sort of thing. Here is a short snippet from the company's website:
WordRef is a Canadian-based, online educational resource that specializes in helping university and college students improve their writing skills. It offers a unique and interactive proofreading and copy-editing service and is especially helpful for international students who may be struggling with writing in a second language.
You can learn more about WordRef and its services at www.wordref.ca or on its facebook page at www.facebook.com/wordref
Happy writing!