SMC (sage math cloud) is a great way to get going quickly and efficiently...no setup problems. I used it almost exclusively last year, and my students did use it exclusively. SMC is a full-blown unix env. with dozens of tools if you or your students are interested in other areas of technical computing.
My 150+ students are now actively using SMC. This time last year I was running around in a panicked state rebooting servers and various other things as our local Sage Mathematical Software System server kept crashing. This year it's been completely painless with all the students using SageMathCloud. Even had the opportunity to teach some markdown to some students and also heard of a group of 3 who were collaboratively writing revision notes using a shared +IPython notebook. Great tool.
The LaTeX in SMC is phenomenal and I really love the ability to store all of my files together in a project. It's hard to say how awesome these features are! This is handy for any kind of work; from a teaching perspective, I think this will be particularly useful in sharing information and providing an unified workspace.