okay class, we just went over big-O. here is an algorithm... what do you think the big-O of this is: O(n), O(n^2), etc...
Nothing that you'd have to keep track of the answers for longer than the length of the class.
Plus, if you added logins, that would just make it that much more of a setup / hassle to get classes going.
That said, this is a great idea. This is the kind of real-time feedback that I would love to have access to. Many of my students would rather fail than put their hand up and ask for help. Something like this would really help them out.