Because they have to be pretty much trained wizards at the end to win, so it has to be convincing. Also, the events have to stretch over the entire course of study -- and before it, even. (I have to work on segueing over long periods of time. My muse just loves them.) And there are two plots to work into this structure.
Already set up that the more power you have, the more intensively you have to drill the basics before you work on higher order stuff.
Possibly three years -- basic elemental work (each one with an assigned element); working all four elements; starting to combine. Though they will learn more in the field after the story's done.