Alexithymia

A detective investigating a murder occur at an elite high school. 4 personal belongings of the victim's friend found together with the body.
Friend 1 : the top student who is a genius and perfectionist
Friend 2 : takes an interest in photography as an hobby and runs the school's photography club
Friend 3 : the High School's "queen bee"
Friend 4: the school's self-proclaimed idol
So, which one is the killer? or the four of them is being set up by someone?