Awesome! I love that poster! I'd totally go join that school if it existed (and if magic were real and everything ). I have trouble with some school subjects, but I've studied hard and long before, so I could do alright if I tried.
Perhaps, though it matters what Shaman we are going with. D&D Shamans, WoW Shamans, or some other type. The most common idea being that they are more of a feral/barbaric/tribal priest then controllers of the weather.
What is your definition of Shaman and/or which are we using?
It just strikes me as unnecessary. Pegasi cast magic through their wings (and possibly hooves) and earth ponies likely exclusively through their hooves. However, that is the only significant difference, unless you want to make the whole thing stupidly complex for no good reason.
You keep on making the same error and repeating the same assertion. Yet, not even unicorns have the flexible magic that you imagine, outside of some very high-level adepts like Twilight Sparkle. Instead, we see them having a general power (telekinesis) and a very narrow set of specialist spells entirely relating to their cutie mark 'true talent'. It takes time and training to move beyond those limits. Similarly, it would take time and training to teach a pegasi or earth pony to go beyond the fairly narrow band of abilities they generally show.
That is, of course, what Twilight is probably trying to do with her new school of magic.
This actually seems highly doable to me. Considering the whole Cutie Mark phenomenon, I think it's possible that all ponies have a little magic in them somewhere. Could be that a unicorn's horn is just the conduit.
If anypony could figure out how to tap into all that hidden potential it would be Princess Twilight Sparkle.
All ponies DO have magic. Lauren Faust says that while unicorns are the only ones who can consciously control their magic, pegasai and Earth ponies have a "passive" form of magic. This is why pegasai can control weather and walk on clouds, while only Earth ponies are able to grow food and, as Lauren said, "have a deeper connection with nature." The alicorn princesses possess all three forms of magic.