William had his wand out, not really thinking of anything but threat, and only discovered when he rounded the corner that the threat (or at least the reason for the screaming children) was Pete.
"It's all right," he insisted, taking charge of the students and leaving Pete to deal with the enormous predator in their midst. "It's just a Patronus. You've all learned about the Patronus charm, right? Can anyone tell me how it manifests?"
So he wasn't a teacher and was generally horrible at dealing with groups of children. Whatever. Desperate times, etc. And maybe if he distracted them, Pete would have an easier time with the lion and the dozen terrified animals currently plastered to the walls.
A rabbit hopped up next to his feet and looked at him with wide, limpid eyes, quivering hopefully. "Er," he said warily. "You belong to a sixth-year, don't you?"