You'll have to correct me of what other 'idiot disciple' we're talking about. On the second to last panel of page 32, the idiot disciple blurted out to the dark elf how she'll take in another disciple 10 years in the future and used that as the reason for why the idiot one should train under her, as well. Furthermore, she called that disciple 'the heroine', something you'd normally hear about in stories or (otome) games, totally out of place in the current setting.
At the very beginning of this series, a crazed (reincarnated) woman tried to kill that same heroine and use a half-baked magic stone with her memories contained in it as a means to insert herself into that girl's body, becoming the heroine, instead. That stone was mentioned again for an entirely different purpose as explained in page 34 of this chapter.
That (reincarnated) crazed woman who got killed by the heroine instead and the 'idiot disciple' were the same person. Furthermore, the stone that was left behind got picked up by another girl at the end of chapter 3, and it seems by that time, that reincarnated woman has taken control over her body, instead, but that's my speculation for what happened, I'm not entirely sure, myself, but that's about as 'alive' as I could make her out to be.
So far we have the 'sturdy disciple' currently under the dark elf (the heroine/MC), and the dark elf's previous 'idiot disciple' as I've explained here. That's 2 disciples we've read about so far. If there was a 'different idiot disciple', please explain to me who that is.