Talk:Trial of the Grasses/@comment-2601:2C1:C280:228B:D5F9:BE4B:668A:B320-20180510162802/@comment-8742750-20180510180637

Blood of Elves seems to suggest that the Wolf School witchers have the herbs, Grasses and Laboratium needed to create new witchers but not the sorcerers required to actually put it into effect. It doesn't necessarily seem to be a case of being unable to; rather the witchers are so protective and wary after the previous assault that they vehemently refuse to let another sorcerer, any sorcerer, near their secrets ever again. Indeed, Triss implies that she herself might have been able to conduct the Trials if she were given the chance, but even she, one of the most trusted magic users amongst the witchers, isn't allowed to see their secrets.

As for other schools, there is barely any indication of anything from them in the books. In the games, if the Viper School is anything to go by, then they didn't fare much better during the attacks, as I believe it was suggested they too were subject to an anti-mutant attack. Guess it isn't impossible though, as we don't really hear enough about the Bear and Maticore, for example.