The seal on the floor of the mages guild (in OB) has the first three letters, in daedric, of five of the six schools of magicka. There is no restoration. I was trying to come up with a lore-based reason for this, and the best I could come up with was the fact that the mages guild has Psijic origins. Perhaps the PSJJJ aren't interested in restoration, since the aedric temples have a bit of a monopoly on that sort of thing. To be sure, the mages of the guild study restoration, and can teach it, but it's not their original intent, at the time of founding. That's the best I could come up with. Anyone wanna' take a crack at it?

I have absolutely no idea why it's not there but.

Mysticism AKA the Old Way can if the Mage has enough concentration and focus on it act just like any school of Magic.

So that could be one reason... due to the fact that Mysticism could be(come) any kind of Magic.

That's kinda' what I assumed. The seal itself probably pre-dates the guild as we know it today and is based on Psijic principles. Why would a psijic need restoration?

Indeed when he have the Old Way.

Wait but then why would they include the other schools if that was the case.

Schools are arbitrary. Maybe Restoration wasn't a full-fledged school, but merely a branch of Alteration at the time they made that seal. After all, the School of Thaumaturgy disappeared, subsumed by Alteration.