This.
You'd be surprised what kids pick up on, even from an early age. Given he's 15 years old, he's probably had a life of what he himself experienced, or at least seen it happen to other people.
Bear in mind we've only seen a very short clip of what he's been through at school. If he's been there a while, I can guarantee he's gone through days and weeks of similar treatment, verbal if not physical.
Sadly, this is one child amongst thousands in Huddersfield, and millions and millions in the rest of the UK that this happens to. Obviously some schools are very good at actually doing something about it (bullying in any form) so it's nipped in the bud early. Other schools are bit slower to react and then it leads to what we see in the video. I would love to know what the school plans to do about this incident, though. It's very violent (strangling and water-boarding ffs) - things like that don't happen overnight so I'd bet the school knew there were issues with this kid.