A good website for this is http.//figment.com/
I am not very familiar with the writing communities, but the benefits can be remarkable. This community of writers allows students to take academic risks. It also is kid friendly, and so, students will not feel intimidated because they will be able to work with their own peers in a state or national level. With the advent of End of Course Exam, which requires students to write creatively, these community of writers can prove to be a very good way for students to see what other students around the state and nation are doing, and thus increase the rigor through exposure.
Other websites where students can communicate with other writers and get feedback: