When the doors slammed behind Granger (except maybe not Granger?), the Great Hall stayed silent for several seconds before harsh whispering broke out.  Draco was a little unclear on what in Merlin’s beard just happened, so he stayed to the side where he and his parents had been quietly sitting for a few hours.  No one had explicitly said "don’t leave."  However, the wandering glances from the surviving Aurors told them enough to stay in place.