Apparently, Witcher schools were never meant to be a thing. Try telling that to CD Projekt Red, which has poured this lore into the world of The Witcher, highlighting the differences between the schools of mutant monster hunters. However, according to the original author Andrzej Sapkowski, the idea of Witcher Schools was something he never meant to create.
"A single sentence about some 'school of the Wolf' mysteriously made its way into The Last Wish," Sapkowski wrote in a Reddit AMA (thanks, PC Gamer). "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."
"Adaptors, particularly videogame people, have clung to the idea with remarkable tenacity and have wonderfully multiplied these 'witcher schools.' Completely unnecessary," Sapkowski wrote.
Sapkowski went onto explain that while he thinks the games and TV series based on his works are fine, the original will always be his preferred representation of The Witcher. That's the one without schools, just so we're clear.