[lowes96]

I think the janitor killed him remember their were wet signs in the bathroom and the kid went in a stall that was out of order. When the janitor came back he got so enraged by this kid making markings in the out of order stall that he killed him by drowning him in the toilet and hiding his body in the gym. The janitor will reveal himself as the kids break into the school after hours. But Christa will transform into a werewolf and kill the janitor avenging the boy's death and allowing him to rest in peace. The janitor is seen in the videos of the CCTV cameras in the school he is wearing a blue uniform and black shoes which we see when he murders the boy!

PS thats who i think killed the boy