How I Use It
It is good to use as an archive for lesson plans, homework, quizzes, tests, etc. Dropbox is a cloud based file storage space. Use in the classroom as an interactive tool is not recommended. If one were to use it for students to hand in work, it would be a hassle to set up with emails and permission settings.
To use it as a collaborative tool (where a class can edit a file similarly to a wiki) will prove to be a lesson in futility. Perhaps use something like MS Office 365 or Google Docs for something like this.