Ignatius crossed his arms over his chest. He moved to look at whatever it was Mitchell was doing. He frowned a little, "You are..." He frowned even more. "Ten points from Slytherin, Mendel." He hoped he wouldn't need to turn it into a detention as well. Hopefully the other boy would take his things and go back upstairs.
"I suggest that you take your posters and return to your bed. I am sure you are distressed about the current state of affairs and I would not wish to have to give you a detention, but as Prefect, I cannot condone such behaviour." Hopefully that was polite enough that Mitchell would agree and be on his merry way.
Otherwise, there would likely be a detention as well as the ten points now gone from his house. Certainly he wouldn't want them to lose more points, right? Surely not.