Maybe it's because I teach in middle school (me? jaded? nah, they're all darlings), but it seems to me that he was intentionally messing with you and found out he took it too far. He misjudged his prankster ability. Yes, a fifth/sixth grader (11/12 years old) is perfectly capable of that sort of "doofusness". Don't beat yourself up, and I'm glad everyone's OK.