The notion of various witcher schools arose from an unintended remark made by Sapkowski, which he later deemed narratively flawed and unnecessary. He noted, “A single sentence about some ‘school of the Wolf’ mysteriously made its way into The Last Wish. I later deemed it unworthy of development and narratively incorrect, even detrimental to the plot. Therefore, later I never included or referenced any Witcher Gryffindors or Slytherins again. Never.”

This statement emphasizes that the unsuspecting roots of a widely embraced concept can often sprout without a solid foundation. Despite Sapkowski’s attempts to dismiss the idea, it has been perpetuated, especially among the gaming community. “However, that one sentence was enough. Adaptors, particularly videogame people, have clung to the idea with remarkable tenacity and have wonderfully multiplied these ‘witcher schools’. Completely unnecessary,” he remarked.

Contemplating the future, Sapkowski hinted that he might remove the problematic sentence in reprints, but also suggested the possibility of expanding the lore around Witcher medallions, saying, “The sky is the limit.”

For more context, you can find the complete discussion here.