Not quite what I was expecting, and at this point it doesn't really make sense yet.
I figured they were going to be transferred to DHS so they'd become Mana's problem. And that is still possible if they aren't allowed to come back afterwards.
Even so, you'd want to be upfront about it. Sending them down into a tunnel that's been abandoned for 200 years doesn't exactly strike me as a punishment in the usual sense, unless the idea is to dispose of them by means of falling masonry, which seems rather excessive. Either way, they're clearly not being told the whole story here.
Honestly I was expecting a transfer too, since rumors might make it seem like Mana is more capable of handling trickster students... I wonder if the Academy or O'Reily know that they presently have 9 students. ...who is the Academy headmaster and when did he meet Mana?