It's silly, but I think that this is just because any calculator that is more useful would also make cheating too easy and would also provide too much assistance. Teachers know what the TI-84 can and can't do, so they don't have to familiarize themselves with the risks of any new model. The price is ridiculous though, I'm quite sure that Texas Instruments makes a mighty profit on these things.

I loved the TI-84 when I was in highschool. Instead of doing whatever I was supposed to do, I was programming silly things on the calculator. It had quite some programming functionality, and I constantly kept finding new and interesting things to do with it. It played a part in me studying computer science, which the teachers recommended me to do because I was constantly programming on the calculator. I still have it, though I currently don't have any batteries for it.