No, you assumed that something was known from the start with no indication. I make it clear that I am assuming and that there are many possibilities but you hide it, pretend it is "fact," and try to make points based on your assumptions.
It's just like everyone assuming that the police shooting an "unarmed" suspect is clearly unjustified simply because said suspect was "unarmed," never mind that it could be that the suspect was actively trying to take the officer's weapon to shoot the officer. :Oh noooooooo. If that fact wasn't known from the start then it's just an "assumption" and the shooting was "clearly unjustified." 
 
If your opinion is that possibilities aren't worth considering because each is an assumption until we know otherwise, then Let me just say that you are a murderer and a child rapists until we get some facts that contradict, even if things don't add up with that assumption in the mean time.
Back to this again. Ugh.
It may be. It may not be. It depends on the kid's intentions. Durr. Don't play dumb.
If it was built to look like a kid's idea of what a bomb looks like, it's a hoax bomb, whether the kid 
claims it's just a clock and that he never intended for it to look like a bomb or not. As others have said, there's very little more he could do to make it look any more like a kid/movie's idea of what a bomb looks like. Any more and he would lose deniability, which may be precisely why it does not include a pipe or fake dynamite or a ball of Play-Doh with wires in it.
No matter what the kid said (unless you think kids never lie to get out of trouble), the teacher suspected that he built a hoax bomb which is ENTIRELY REASONABLE whether it "is" or not.
It very well may not have been a hoax bomb. That's not the point. The point is that they had reason to suspect it was and that reasoning does not apply to anyone walking around with cellphones. The kid may have even been following the first teacher's suggestion not to talk about it, which would have made it even more suspicious to others. Regardless, it was appropriate to treat it as a POSSIBLE hoax bomb and that is exactly what they did. I will refer back to this the next time you ask me how it "is" a hoax bomb in your futile attempt to make a point. MOST confirmed bomb hoaxes don't even go this far (called in; suspicious device doesn't exist). You can't ignore the possibility while making assumptions to dismiss it.