I suggested Constitution as a good stat to base Polymorpher on. I don't think Larian will go for that though because CON is so good. (I imagine that they'll just use INT for both because they like magic, they just don't want mages to be able to cast much.)
Another alternative is to not have the entire school to be based on and get bonuses from one specific attribute.
Instead, have the different forms (skills) Polymorpher has to be based on different attributes, for example the power of your "Transform into a guy with axe hands" skill is based on your Strength, but the "move really quickly and dodge well" skill is based on your Finesse, and your "Become a huge wall of meat" is based on your Constitution" attribute.
The Polymorpher school itself could give bonuses to how many turns you can remain in a form or something.
