Stupidity: There's no reason whatsoever for Mr. Walker to trash the other rooms of David's partners. Someone as tough-minded as Mr. Walker would have been able to recognize the consequences of his behavior. He could have simply trashed David's room and let it be. It simply seems like a plot device to justify David's gang attacking the Walker residence.
Stupidity: In real life, David beating Gary would be a very big deal since it happened in front of many witnessed and on school property. He would at the very least be arrested or detained sooner or later, if not immediately, for things like assault and possibly reckless endangerment, and could very well have a lawsuit filed against him.
Answer: Why was there a diver in the water at the end? Looking for someone's body?
The tide came in and took David's body away. The divers were trying to retrieve it.
No, the end was altered over the years. "Mandela effect" and all proof wiped out. The end has the family walk out to nobody.