Incidentally, he also (next chapter) demonstrated Imperio on students, teaching them to overcome it. He got (or so he claimed) permission from Dumbledore for this, although I don't know why Dumbledore had the authority to give such permission. He also gave them the opportunity to leave first, but taunted them so that they wouldn't.

I assumed that this is how Quirrelmort will teach it here.

Here I was thinking he would students with detention run back and forth across the quidditch pitch, like those ducks in the side show shooting ranges. Put them way over the other side of the field to give them a fighting chance. It should only be the slow and uncoordinated ones that can't dodge in time... what use would Quirrelmort have for dead weight like that?