Normally I'm not a fan of children as characters, major or otherwise. But generally Billy managed to sound very realistic and not annoying. Which isn't the same thing as saying he didn't annoy everyone around him with the typical boundless energy of a 6 year old.
His thought pattern felt very real to me. He sounded like he was going a mile a minute and he'd often start on one topic before offering a random animal fact (he's absolutely obsessed with animals).
The whole story is about a simple misunderstanding and how everyone acts (generally) with the best of intentions and yet everything still gets screwed up. That, too, felt realistic.
It also shows the problem of complete strangers trying to evaluate someone in five or twenty minute blocks of time. Someone who knows him well would probably know about his predator/prey animal obsession so that language wouldn't be particularly troubling. In fact, I'm not sure why the teacher didn't know about it. I would think that sort of thing would reveal itself in the classroom. Certainly the art project he's working on suggests that it does.