It is still happening. I think it happens when the students use a Temporary Copy rather than saving a Permanent Copy. I give them this link
They play around with it in their rstudio.cloud account with 'Temporary Copy' still shown in the top nav bar, and the time they spend playing with the Temporary Copy gets put on my account.
I still have 2 weeks before I'd like them to work in RStudio Cloud for a homework problem, but need to get this solved or else I'll just have to demo it. I have 100+ students. They only need to use about 1 hr, but they'll quickly burn through my hours. I'll tell them to save a permanent copy, which will hopefully help, but if any forget, they'll quickly burn through my hours.
I'm thinking as a backup, I'll make a dummy account and have the workspaces that student use there. That way they won't lock up the account that I am using for teaching by blowing through my hours. First I'll need to burn through hours on the dummy account and see if students can still use the link to the workspaces on a free account with no hours left.