Zero tolerance of anything that disrupts school. Maybe you haven't noticed but there has been a rash of school arrests of preteens (meaning cops are being called at any disturbance), pre-teen kids threatened with expulsion by making a gun with their fingers, elementary school kids being threatened with expulsion for a "paper gun" (imagine a tic-tac-toe grid with a corner square missing), wearing an NRA shirt, and a whole host of other sad situations if you want to search for such things.
Let's not forget that schools have been targeted as well so teachers, administrators, and guards are already very sensitive.
As for the clock, no clocks aren't against school rules, but dude, come on now, that was the shittiest clock I've seen in a long time. That wasn't something analog, or even remotely looked like a clock. It could easily be confused as a bomb by a a school worker who has to be on the look out for such things because once again WHO BUILDS A CLOCK IN A BRIEFCASE? It had giant numbers, a large circuit board, wires EVERYWHERE. Seriously, teach that dude to cut some wire and solder.
I'm not saying what they did was the right thing. They over-reacted. But again WHO BUILDS A CLOCK IN A BRIEFCASE? It's like someone building a telescope on to a rifle stock for portability and stability.
Everyone has to be cognizant of the date and times we live in and there are just things we can't do now. I took a bayonet to school to show my friends. No particular reason other that's where and when I could show it to them. Certainly can't do that now.
So yes, they over reacted. But he acted stupidly as well. Again, time and place.