Yeah, I could see if he was 8, taking an electronic component, stripping all the insides and moving it into new box and plugging it in sounds very plausible. But doing it 14, and then using what looks like a suitcase, where more than one adult thought it was a bomb, makes his story that it's just a "clock" a bit hard to believe.
Moreover, the first teacher he showed it to, urged him to keep the devices in his backpack and not take it out [1]. But he didn't, so that indicates he wanted really hard to get a reaction, and he set the alarm off in another class. Another strike against their story is the speed with which they demanded $15m from the school district.
[1] His engineering teacher, upon seeing the clock said, "That's really nice", but advised him to keep the device in his backpack for the rest of the school day.