> What happens when I have more than one API key associated with the administrator account?
If you mean more than one API key for the same free tier instance of IBM Watson Assistant, then I'm afraid that doesn't trick IBM Watson's systems into letting you create more than five models. They still apply the same limit irregardless of how many API keys you create.
If you mean you plan to create a second free tier instance of Watson Assistant, and create a API key for that second instance, then yes - that would support creating up to ten models.
> What's the best practice if I have approximately 60 students?
Using a managed class account - ref
Kind regards
Dale