]Short sheeting
A prank done at boarding schools, camps or on excursions where children sleep in full beds (also common in the military). It is achieved by untucking the foot end of the bottom bedsheet and wrapping it around to the bed opening, making it look as if it is two sheets (the base plus the covering sheet). The victim will find that he cannot get into bed (as doing this "shortens" the bed length). Known as an 'Apple-pie bed' in the UK.[1]
In a variant of the 'apple-pie bed', described in "The Reverent Wooing of Archibald," a story by P.G. Wodehouse, the upper and lower sheets are sewn together and a spiny plant inserted in between to keep company with the victim as he slips himself in between.

this whole wikipedia article on pranks is pretty funny
school pranks