I think she's just supposed to be an aloof character. it's just that the author doesn't understand the difference between aloof and purposeful negligence. I think it just comes down to the author being shit at writing. There's a difference between a mentor character purposefully letting the main character deal with an issue alone to help them grow and a mentor character just not giving a fuck and letting potentially hundreds of people die for no reason.
She's plainly not just an aloof character. She knows all this information, she was able to obtain the protein powder — a
relic, I will remind you — via a "state secret", and so far seems to be able to act as she pleases. This is neither aloofness nor negligence, but rather a typical trope in shonen media: The mentor having total confidence in the abilities of the protagonist to thwart the villain.
She is unconcerned with the possibility of any students dying because she has observed all the variables and concluded that it won't happen, because it can't happen. Is that negligent? Perhaps. But what else is she supposed to do. Tell the King? Guarantee he already knows the demons are trying to return. Tell the guards or even the army? To achieve what? Knowing that the demons will make a move on the academy doesn't do much good if you don't know what move they're going to make. Try to intercept without enough information, and they'll just be alerted and change their plans, and not only have you not made progress, you've actually gone backwards because you tipped your hand and they'll put more effort into stealth.
Even if they don't, the guards and army are
too weak to do anything: The only outcome will be dozens of guards
and hundreds of students dead.
Doing what the house mother is doing here by putting her full faith in the musclehead to handle everything is literally the best option available that will result in the fewest casualties.
And, as I said previously, the fact remains firm that all of this is Laius' fault because he couldn't build a functioning society.
