I'd probably think it could be used for a bomb. I'd be looking for a blasting cap or some other kind of detonator, and a main charge before I was ready to say it is a bomb.
From another article:
Ahmed was suspended for three days for bringing the clock to school, but police said on Wednesday that no charges will be filed against him. Ahmed’s story has received a great swelling of support on social media, including from Hillary Clinton, who tweeted, “Ahmed, stay curious and keep building.”
It doesn’t take very much c4 or simtex to make a pretty good explosion. A blasting cap is only about the size of a pencil eraser. If they didn’t call out the bomb squad, it was probably fairly obvious that it wasn’t dangerous, but just leaving something like that sitting around unattended would probably shut down the school for the rest of the day.