Consider - an app that replicates this functionality on the phone, but tracks if the user at any point closes the app. This then is reported to the teacher so the teacher knows if there was any cheating.
The used market must be absolutely massive, larger than any other piece of consumer electronics. No wonder TI can't budge from their pricing. These things also do not break. I've left mine out in a blizzard. You throw them at a wall and you will find your ti84 sitting happily embedded in a crater in the drywall.