ANDY'S ANSWER: I have no idea what the average speed of a teenager texting is, but I did find a free app that will test that. It's call ITextFast. You can get it here.
You can also have them text the following message and time them with a stopwatch. The Guiness Boook of World Records says that a woman in England holds the record for texting this sentence in 25.94 seconds. The previous record for texting the sentence was 35.54 seconds.
"The razor-toothed piranhas of the genera Serrasalmus and Pygocentrus are the most ferocious freshwater fish in the world. In reality they seldom attack a human."