Share files between student and teacher (x-post from Ipad)

Has anyone found a good way to share files between students and their teacher? I am trying to find a way for students to share files with multiple teachers for different classes.

I have tried a few different things: CloudOn, Evernote, DropBox, GoogleDrive. The problem I'm running into is there is not a good way for students to turn in their work to their teachers. DropBo and Google Drive require editor access to the folders, but this enables students to edit other students' work. CloudOn allows us to email files, but I haven't gotten it to work where we can send files to the teachers and have it organize and sync into CloudOn. I am currently looking into WebDAV but I'm not sure what all apps can use it.

Anyone have any other no-cost sugestions or any low-cost ones?