I was thinking of a small IR/LDR based intruder alarm (kids will have fun putting it up in their rooms) any other such suggestions which kids can relate to and apply in their own worlds?
This reminds me of a project back when I was in grade school (5th grade); my friends and I were constantly missing things (pencils and paper, mainly) from our desks (assigned seating, old-school lidded desks), we suspected the teacher.
I ended up rigging an alarm using my 150-in-one kit from Radio Shack and a custom trip-wire like switch to the desk lid; it could be turned off if you knew what to do by carefully openning the desk, reaching in, turning it off with another switch.
Imagine our surprise when, during recess (do kids even get recess anymore?), the alarm sounded very loudly and our teacher ran out calling my name to come shut the thing off. I turned it off, looked at her, then went back out to recess. No other words were exchanged that I remember.
I was never sent to the principal.
Our desks were not messed with the rest of the year.