I've always found language usage and how it changes over time/in different circumstances fascinating. There are two types of linguistics: prescriptive and descriptive. Prescriptive lingustics is used to teach people how they should speak, and descriptive linguistics is used to describe how people actually speak (http://www.linguistics.ucla.edu/people/schlenker/LING1-06-LN-1A.pdf).
Many people notice a significant difference in how people are "supposed" to use language (prescriptive), and how language is used, particularly in social media (descriptive). A conversation I came across on Tumblr discussed this in detail. First, someone wondered why there was no punctuation on Tumblr; Another person added to the discussion by saying how things are written is a way to communicate tone in a toneless medium: http://25.media.tumblr.com/239602560e3e1c2f8657272872aaea0a/tumblr_mjpqwssb7f1r1moueo2_1280.jpg
I think middle and high school students would find analysis of writing style used in social media, and how it differs from formal language styles fascinating. It would probably be really thought-provoking and give them a deeper understanding and awareness of how language is used in different situations, and how language usage evolves.