It used to be that all a pollster had to do was to call a number of people and interview them to get an accurate poll. With the advent of cell phones, caller ID and online self driven polls, a scientific poll is becoming harder to find. This article from one of my favorite blogs, 538, gives a very detailed explanation why polling is so difficult.
If you want an idea for a class, I would find a poll and have your students watch the video below at home and then come in and have a series of questions to be used to answer the 538 article, remembering key terms such as scientific, straw poll, Literary Digest, sample, etc. that you might want to cover.