Instead, this is the picture of the device (from the police department):
It's in a "pencil case", not a briefcase. You can compare the size to the plug on the right.
They didn't think it was a bomb, but a "hoax bomb". If they thought it might be a real bomb, they would've evacuated the school. Texas has specific laws making illegal to create a hoax bomb -- it is for breaking this "hoax bomb" law that the kid was arrested.
This changes the tenor of the discussion. It wasn't that they were too stupid they thought it was a bomb, it was that they were too fascist believing it was intentionally a hoax.
These questioned him, and arrested him because his answers were "passive aggressive". This is wrong on so many levels it's hard to know where to begin. Of course, if the kid's innocent his answers are going to be passive aggressive, because it's just a clock!!!
It was the english teacher who turn him in. Probably for using a preposition at the end of a sentence. The engineering teacher thought it was a good project.
It's actually a sucky project. He didn't build his own clock so much as put existing parts of a clock together into a box.