Who are 'they'?

In the next Harry Potter sentences, who are ‘they’? :

There was an uproar. It took several purple firecrackers exploding from the end of Professor Dumbledore's wand to bring silence.
"Prefects," he rumbled, "lead your Houses back to the dormitories immediately!"
Percy was in his element. "Follow me! Stick together, first years! No need to fear the troll if you follow my orders! Stay close behind me, now. Make way, first years coming through! Excuse me, I'm a prefect!"
"How could a troll get in?" Harry asked as they climbed the stairs.
"Don't ask me, they're supposed to be really stupid," said Ron. "Maybe Peeves let it in for a Halloween joke." They passed different groups of people hurrying in different directions.