When Conor and his dad have a conversation in the car, Conor's seat belt is on at first, disappears, and reappears a few times between shots.
Conor is in the same school class as Harry (the bully), who looks three or four years older than him. While it might be explained that Harry has been held back repeatedly, such a statement is never explicitly made. However, children are rarely held back in UK schools. It's more likely that Harry is either tall or this is a special class on a particular subject that includes children from multiple year groups.