MILFORD, Conn. — Jonathan Law High School has not known much tumult. Students here are often too involved in sports, music or volunteer work to get in much trouble, staff members said, and the biggest source of friction is the annual game between the school football team, known as the Lawmen, and its crosstown rival.

Chris Plaskon seemed to thrive here. The third of five brothers, he was a joke-telling wide receiver on the football team who also played baseball and ran track. Though not immune to the stresses of adolescence, teachers and friends said, Mr. Plaskon showed little sign in recent months that he was troubled.

But a day after authorities say Mr. Plaskon, 16 and a junior, fatally stabbed a classmate in a school hallway, teachers and students were struggling to make sense of the incomprehensible: how a student whom many described as funny and popular could suddenly be accused of killing Maren Sanchez, 16, a well-liked honor student and his longtime friend.

“They’re looking for the kid in the black cape and the fangs and the black fingernails, but there was no sign,” said Mark Robinson, 38, who was Mr. Plaskon’s football coach before retiring last season. “He wasn’t a kid who was in the shadows. He was a well-liked kid. He was funnier than hell. That’s what makes it really strange.”