These are two great apps for the upcoming school year. The Android one is free and can also be used on a PC. The Apple app has ads that you can get rid of for 99 cents.
This app is only for Androids and PCs:
The Wabbit graphing calculator emulator looks like the TI-83 or TI-84 and is supposed to mimic its features. It can be used on your phone or your computer. It's been around for a long time.
This app is for iPhones:
I don't know how close its features are to the TI calculators, but a lot of people with iPhones really like it.