What is the sound of a student learning?
Is it the teacher talking?
Is it the No.2 scratch of compliant pencil on paper?
Is it the silent regard of sagacious lectures?
Or is it a soft, murmured discussion with another learner, even in disregard to the "don't talk when teacher is talking" rule?
Or is it the face of surprise, concentration, and reflection?
Maybe it's a loud, impulsive burst of information that seems disconnected to everyone and everything else.
Or maybe, it's a question.
For strategies that work with secondary students, take a look at Transparent Teaching of Adolescents: Creating the Ideal Class for Students and Teachers.