tell them that detentions contravenen the human rights act and that if they keep u there they are infringing all sorts of really important laws.
or tell the m that u'll scream until ur sick if they make u stay.
or tell them that u have a date wit ha three legged ostrich named bartholemew, a pixie, a washing up bowl, a mouse called gerald and gary rhodes. u'll then be certified mad, and not have to do detention.
of course, u will be in a nut house, but u can esasliy escape from there using ur fire breathing gift. thats what i didi anyway.