After reading a few responses I did want to clarify one minor thing; a spell’s “School” is not the same thing as its Defense Category. There are spells with the school of holy/shadow/frost/etc. that have a defense category of melee/physical, and spells with a school of physical that have a defense category of magical.
…and before it’s asked; no, we are not going to post a full list of which spells are in which spell school or defense categories 
I’m also going to go ahead and close this thread since we’ve concluded our investigation into this issue. We very much appreciate everyone’s efforts to help us track down inconsistencies between WoW Classic and the original game.