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.
Granted it's been awhile since I set any charges, but I've never seen a blasting cap anywhere near that small.