It is indeed true, that alteration and transmutation have some similar spells. The biggest difference however is, that alteration is 90% about quick battle buffs, whereas transmutation permanently improves or lessens coms/units stats & abilities with ritual spells. So more likely it goes somewhere between alteration and construction.
Personally I like this school, because it has so much variety. You could make an army of flying knights or turn Jotun giants into trampling nightmares. There are endless possibilities.
I also have one thematic improvement for this school, which gives additional excitement:
Each spell in transmutation has a very small percentage of failure (5%?). This would cause something very random to happen (e.g caster grows two new heads or goes hostile). This naturally grants even more hilarious moments to be written in Dom's history.
