i don't agree with the assumption that they are unbalanced. fire is definitely the best school for tactical damage spells, that's obvious. but that doesn't make it supreme. other schools have other purposes. earth gives you awesome buffs (hammers/set in stone for city production, stoneskin/natures grace [? not sure about the name]/giant form as unit buffs, tremor and earth quake as strategic spells; water gives you slow, pandemoinum (very strong when you make it instant with the savant perk), inspiration, freeze, tidal wave; air has haste, tutelage, propaganda, cloudwalk and tornado, life has shrink/growth, crusade, call to arms, wellspring; death has wither, curse [seems to be broken atm, but when it's working again, it's really powerful], mass curse [same as curse], blind, sacrifice, kill.
i've played sovs with all kinds of spell school combinations and i honestly never had the feeling that one of them was not solid. sure- a life/air mage sucks if you want to play them as a destruction mage (evoker), but can be really awesome as a support unit for an army (shrink strong monsters, heal wounded soldiers, cloud walk their stack across the map instantly, tornado a large stack to single out the dragon without having to kill his army of pack drakes. earth spellbook isn't much use for a mage, but it's a great addition to a non mage character etc. etc. (buffs and strategic spells don't require prodigy or evoker ranks to be useful) etc etc.
some of the spells could use some tweaks (scaling etc), but overall, i think they are fairly well balanced actually.