i think study hall is a waste of time - it is "baby-sitting" - and i had one study hall throughout all of high school...and i never stayed in that study hall...
but...
students enroll in study hall - and teachers are paid to maintain the rules of that study hall.
it might be a "waste" of tax dollars - i would rather that study hall be a horticulture class or a world religions class - but schools can not afford that...