Ios longevity on different ipads

[Monkt]

I'm trying to raise money for iPads for my science classroom. I want to get something that will really last. What iPads should be supported by future ios releases longest? What determines this?

 

[bigchrisfgb]

The latest ones.

All models currently on sale will run iOS7, future version maybe not though I think they will all see through iOS8 (though with limitations).

What dictates it?, the chip and what it can handle and if Apple have a new key feature on newer models which they want on only certain models in order to sell newer models.