Like I said, it's a reenactment of the next-to-last scene from the movie Kazoku Game (The Family Game). The guy that beats them all up is the tutor hired to get the younger son's grades up so he can get into the elite high school that his parents want him to go to. That sort of situation (the tutor, not the foodfight was and still is a common thing in Japan, with a huge amount of stress placed on children to not only succeed but to excel academically. It's a very neat movie, and I'd definitely recommend seeing it if you get the chance.