Doesn't an IED usually have some component that actually, like, blows up? Could it really be that hard to determine that there was nothing in this case that would explode?
Well, in the school's defense, the homemade clock was an "improvised device," and "ID" is just one letter away from "IED," so.....yeah.
That's true. All this little 'clock' needed was some explosives.
Ever heard of this thing called detonation cord? (You've seen it several times actually being detonated if you're a regular viewer of Mythbusters.) A detonation cord is a thin, flexible tube shaped to look like a wire filled with plastic explosives (US military special forces use them a lot to blow open door locks); just 10 to 12 inches of "det cord" is enough to create an explosion more powerful than a standard stick of dynamite. Ahmed Mohamed's homemade clock could easily have disguised a "det cord" as one of the wires in that clock, and when the timer goes off, you have a pretty powerful and dangerous improvised explosive device (IED).
Given the very heightened awareness by teachers and school officials of suspicious activity after the 1999 Columbine High School tragedy, it's not surprising that the teachers and police wanted to make VERY sure that "clock" was a real clock and not an IED disguised as a clock.