Do schools have the right to make detention on a saturday???

okay well i just got my schedule today for sophmore year and with it came the usual papers that the school sends but as i was looking through them i found a paper stating there would also be detention on saturdays...and i got really upset..and thought does the school have the right to make students go to detention on a saturdau when we only go to school monday through friday..do you think its fair they are doing this?? i was just wondering what everyone else thought about this. i mean i think its wrong if they want to give us detention it can be before or after school not on a saturday..we only get saturdays and sundays off from school.