is an extension of it's true meaning. someone who polices the halls and rats on fellow peers for petty power, self esteem and approval from elders. someone who gets off or generally thinks they are in charge by randomly busting you. self appointed judges of anything.
who do you think you are, the fucking hall monitor?
Having sex in or around school grounds.
John:I totally fucked this chick in the science lab!
Joey:Oh so you were a hall monitor.