What I mean:
Okay, but why did Dumbledore tell the Prefects to take everyone back to their dormitories to keep them safe while a troll was in the dungeon? You already had (almost) everyone accounted for in the Great Hall. One singular room is way easier to secure, and you wouldn't be risking student lives by sending them back through the school, especially the Slytherins whose dorms are IN THE DUNGEONS?!