Who was the killer in movie Detention starring Josh Hutcherson?

There is no movie called "Detention" starring Josh Hutcherson. There is a 2011 horror comedy film called "Detention" starring Josh Hutcherson, but the killer in that film is not a single person but rather a supernatural entity known as "C.U.D.D.L.E.S." (the acronym stands for "Crazy Undead Demonic Demonic Lunatic Evil Spirits").

It is possible you are thinking of another film, or perhaps you are confusing Josh Hutcherson with another actor. Please provide more details if you can remember, so I can help you identify the film and the killer.