Balagarn's Horn is listed as a transmutation spell, castable by wizards as a second level spell. Yet I cannot learn it with my Illusionist because it says it is from my school of opposition (Enchantment).

It's enchantment, according to the handbook.

It says it's transmutation in it's IG description.

Gee, then you should be able to cast it, huh. HUH.

This is a known issue, and bioware bug. NWN considers balagarn's an enchantment spell, and as far as I know it can't be changed without a hak.