So could everyone else. That's why they didn't call the bomb squad. Let me guess, you can read the kids mind and you know for sure he just wanted to impress his teachers with his incredible ability to place a fully assembled clock into a different box.
It was a clock, unti he removed from a clock looking case and put into a case to make clock that also functions as a hoax bomb. You can't be telling me you think he improved a clock somehow by hiding the display in a closed case and exposing the transformer and making the buttons very difficult to pre.