Its at the scene when Stu, Tatum, and Sydney are at their lockers, Stu walks away with Tatum, and Sydney goes the opposite way when she bumps into Billy, then she runs into the bathroom after their fight and you can clearly see there is only one path to the bathroom but when she's in there there is another killer in one of the stalls, you can see his boots... Stu and Billy are the only killers and they had no way of getting in there- so who was it?