I'm from Australia, QLD. My School Quote; (Stolen from website). We are in-charge of your child while they are in our grounds however you are in-charge of staff. You may exclude your child from class and detention. Although we strongly encourage you to let us keep then in-detention as they have done-so at school and not while you are in-charge of them.
Your guardian has a right, you do not. I learn't that the hard way. Just get a parent / guardian to complain and your off. If they don't want their child in detention then they get it because they are legally in-charge of you.