Actually any school can get from either a amulet, or some type of hat, robe, or shoes, and it as your second school. Really i'm not sure why that like every enemy has the weakness spell.
Wouldn't it make more sense if just balance had it because its a balance only spell? :?
It adds to the thinking a student wizard has to do when preparing his/her ready spells and items. It, in a way, forces us to consider wands other than our own school so we can blow away the weakness, etc.