What would you do if you knew someone who shoplifted?
Someone who stole from someone's locker?
Someone who assaulted (physically) another for no particular reason?
Someone who assaulted (sexually) another who was unwilling to come forward?
Someone who took a car for a joyride?
Someone who committed armed robbery?
Someone who committed murder?
My point: is there some arbitrary threshold you believe exists at which you feel your duty kicks in to lodge a complaint? If so, why do you imagine anyone else is qualified to tell you what that point is?
Why do you characterize this as a moral dilemma, which implies you detect a need to choose the lesser of two evils? On the one hand, you are aware of wrongdoing which should be reported. On the other hand...what, exactly?
At least one question to ask: is this something you know firsthand, or secondhand?