to turn a bookbag inside-out
often used as a prank among high school kids
when you skin some one's bag you take out all the contents inside
turn the bag inside-out
and put the stuff in
usually done near the end of class so that the victim will be so busy skinning his or her bag that he or she will be late to the next class
"Damn skinning a bag is a lot of work but it's worth it."