It looks like his intention was to make it look like a bomb. Even if it wasn't, there's no way that "hey, this looks sort of like a bomb" didn't go through his or his parent's minds.
Money/attention grab. Any other excuse the apologists are making is bullshit. They should have questioned the kid. Frankly, I think they should have suspended him -- and I bet they would if they wouldn't get sued. Guessing the parent's aren't too happy about that one, since it blows money grab plan A out of the picture.
He was already warned by the first teacher (engineering), NOT to show it to others at the School. Hinting that, that teacher either knew it looked like a bomb or even mentioned/hinted at it ?
Yet the boy ignores this, and continues to mess with it, when he should have been concentrating on his lessons, and NOT disturbing other hard working pupils.
It was his further messing around with it (against the first teachers advice), which caused it to make (loud ?), noises attracting the English teachers attention, and causing this incident.
tl;dr
He was being at least a bit bad, if not worse.
Also when I carefully watched his video(s). I get the distinct impression, that he did indeed, realize it atr least partly if notr fully, looked like a bomb. Because he said that he used the external cable to make it look less (can't remember the exact word, but it was smething like) dangerous? or suspicous? or something?.
tl;dr
Was his "invention", a movie prop of some sort ?
Or to put it another way. Who exactly wants a clock built into a brief case ?
But I could be going too far here. There is little evidence at the moment, to show he significantly intended to create a "hoax bomb" or "bomb", incident.