This question is exclusively about Netflix's The Witcher. I know this doesn't happen in either The Last Wish (which I've read) or the Witcher saga, but I'm not interested in how it differs from the novels or whether it's a bad or good change. I'm just interested in its explanation within the TV show.
While Yennefer is training at the Aretuza academy for female mages, we see a scene where some of her fellow students are taken to a room and then
transformed into eels by Tissaia, much to Yennefer's dismay. Tissaia's words imply these students weren't worthy of "ascending".
Yennefer considers at least some of these students her friends. Since I have a hard time telling Aretuza's nondescript students apart, except obviously for Fringilla and Sabrina, it's not clear to me if this was permanent. And also,
is this the fate that awaits all students who fail to graduate/ascend? Were these particular students aware of this? They don't seem surprised or afraid, even when Tissaia isn't particularly fast about it.