Well, I guess I'll go through process of elimination:
Divination...I don't think that even warrants explanation.
Abjuration: it's not really protecting you as much as curing you so I don't think it'd be abjuration.
Illusion: Well, it's not an illusion, it's pretty real.
Enchantment: This generally has to do with the mind and such, not the physical body, so I'd say that's out.
Evocation: Granted, you could be evoking new skin into existence, but evocation generally deals with energies that come and go, so healing wouldn't fit there.
These are the one I think are pretty blatantly out...
Now for the other three: Transmutation, Conjuration, and Necromancy
Conjuration: you could be conjuring new skin/blood/muscle/etc. into existence on their body, and thereby healing them, but conjuration generally doesn't affect a creature, and generally conjures a whole new creature or body into existence. I'd say the other two are a bit more warranted.
Necromancy: This school generally deals with life force, so if it can be negative in dealing with life force, why shouldn't it be able to be positive as well?
Transmutation: This generally involves altering creatures and objects, therefore it would be a good qualifier for morphing the body back into its original form.
Eventually I chose transmutation, but I think Necromancy and Transmutation are pretty close together.