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: I always thought it was a girl giving him a blow job.
Me too! I've seen it more than once and it's not until I'm watching it just now that it shows up as a dog, I could have sworn it was a girl.