First of all let me point out while Alchemy has been in the "magic" specialization in past games that doesn't make it a school of magic.
Except that it in Lore alchemy IS a magic school. :banghead:
"There are seven arcane disciplines in the Mages Guild:"
one of them being Alchemy-
"Alchemy
The Alchemy discipline is the processing and refinement of ingredients and materials through arcane processes to elicit and preserve their subtle, hidden magical effects in alchemical potions.
The center of Alchemy discipline is the Lustratorium in the Arcane University, under the administration of Julienne Fanis (Breton, Wizard)."
BGS do retcon some things on occasion.To date it hasn't been retconed in the actual lore But Alchemy, in every reference and game, is either a mage doing it, or is specifically and clearly labelled as a school of magic. the study of alchemy is in the mages guild, not thief. All the alchemists I know of are mages, none are a thief class. Even in the game mechanic it was governed by the attribute intelligence. The properties of the ingredients are always called "magickal properties" or "magickal effects".