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?