Sorry for the problem opening your project. We believe we have restored it, so please try again to open it.
For additional info, the problem is related to this issue: Issue Opening Projects
Regarding using RStudio Cloud in a teaching environment - we are still in alpha, so there is definitely a chance that there will be occasional issues with the service. That said, hundreds of instructors have used Cloud quite successfully in their courses to date - and our primary goal is to provide a high level of reliability for the service, so most of our current efforts are toward achieving that end.
Hope this helps answer your questions,
Robby