Mark Perry offers this post on Carpe Diem about butter, margarine and opposing interest groups.
I actually believe I had heard this before, but I must have forgotten it. It does sound familiar. It's an excellent example of rent-seeking by interest groups; and Mark brings it around to discussion about trade issues and why special interests can do a better job seeking protection and a beneficial position in the marketplace at the expense of consumers.
I recommend you give it a quick look and see if might serve you as an example for your classes.