Interesting.
I can't find anything in the codex or rulebook which clearly defines "acid=damage type", too.
This leaves the question : " What is acid ?"
Acid is not listed in the codex.
The damage-type entry points to the rulebook entry, which doesn't list acid (as it wasn't there yet).
The expansion rulebooks don't mention acid either.
Corrode says it is an acid condition...but that doesn't mean there has to be an acid damage-type (?).
(web versions, didn't check if paper version is better but am too lazy to get up as I doubt it =P )
I am sure "acid" is meant to be a seperate damage-type though.
So..:
Is acid a subtype of water school?
If so, if a creature is hydro immune, does that mean it is also acid immune?
No.