I found this site about copyright for educators helpful: http://librarycopyright.net/resources/exemptions/
You just plug in what you're doing and it lets you know if it's okay or not. Above all, I think it's important we explain to kids about copyright so they don't accidentally do something they're not supposed to do.
Dropbox is pretty cool. I can see it being useful in that you can add stuff you find on a home laptop or even your phone to use at school for your students.