Benedict is not the type of boy to call negative attention to himself; being a tattletale would require caring a lot more than he does about what his peers are doing, and he's not especially mischievous or easily pressured into bad behavior. So when he goes into Irving's office, he has no reason to think he won't be believed.
"Ser Lutair was in our room just now," he announces, still a little frightened by the urgency, "he was doing something to Colin, and Colin's really upset."