Each desk gets a flic. Students can press to indicate that they're confused or bored, and the professor can get a running tally indicating classroom engagement. They can also put up multiple choice check-in questions to see if everything is sinking in. Lots of options!
I agree with Alex Robar
Alex Robar commented
This is an excellent idea! With the right software, this could make the classroom experience (or really, any presenter/audience experience) much more engaging.