Be careful with cumulative evaluations - how confused students are may not correlate with how good a teacher is. And, if professors are going to be evaluated with the results, they are less likely to use the tool well...

For authentication, I'd make it so that when the professor each session, the class is given a random 4-5 digit code. That way the students would have to be in class to get the code and participate.

You may also want to think about adding the ability for the instructor to add different buttons, not just "understood / confused". You could then have class polls, or group questions to see how many people are paying attention.

This should all be able to be controlled from the instructor dashboard.