According to DudeComedy, the older boy reportedly had a lengthy history of giving the younger boy a hard time before this video, bullying him around.
In the scene shown on camera, the younger guy got backed into a corner by the older, so he lashes out with a punch. The older boy is immediately knocked to the ground.
WARNING: This video contains physical violence and maybe profanity (hard to tell, low quality audio).
Stay in school, kids.