A 12 year old at an FLGS in Sacramento knowing the rules better than a grown teacher is just sad. Four years is a long time to spread bad info to a whole group. Kids read those rulebooks cover to cover, so you gotta respect it.
Kids definitely absorb those rulebooks in a way adults don't, that part is spot on. But the sad part isn't really that a 12 year old knows more, it's that nobody at that table ever bothered to double check the guy running it. Four years of bad calls usually means the group just trusted the teacher because he was the adult in the room, not because they couldn't have caught it themselves. Half the folks at an FLGS have the PHB memorized too, they just don't want to be the one arguing with the DM mid session. So it's less about the kid being impressive and more about a table that never questioned anything.