Throughout my college years, we used these colored cards that were always getting crumpled. Sometimes you lose them, sometimes they rip. Ultimately, think about the amount of trees required to make these paper cards. Given the fact that almost every University (and high schools) I've visited were active users of the cards or a derivative.
So, I come to you with an easy alternative. The video explains the concept. The apps are available in both iOS and Android flavors.
Keep in mind that this is an app primarily aimed at secondary to higher level education classes. The app is purely an interaction tool used by the class to interface with the professor.
Here is the direct link in case you don't watch the video: http://cii.wwu.edu/cii/ABCD/