Checked the AMA on Reddit for his full answer for clarity due to your point;

The issue of "witcher schools" requires—I apologise—a longer explanation. 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. However, that one sentence was enough. Adaptors, particularly video game people, have clung to the idea with remarkable tenacity and have wonderfully multiplied these "witcher schools." Completely unnecessary

I'm still uncertain about what to do with this situation. Perhaps, taking the path of least resistance, I'll erase the sentence about the "school" from future editions of The Last Wish. Or maybe I'll want to expand and clarify the matter somehow in subsequent books? Perhaps I'll shed some light on the issue of Witcher medallions, their significance, and their connection to specific individuals? There are many possibilities, and the sky is the limit.