Now I've discovered an even better use for it. Now, any student trying to surreptitiously text a mate or or do the sudoku¹ will suddenly find their phone or paper illuminated by a red spot, as if an assassin was about to shoot it out of their hands. It's amazing how quickly they put their toys away, and I don't even need to say anything.
I wish I'd had one for school last year.
¹ These free papers have a lot to answer for. Students never used to turn up with newspapers back in the days when they actually had to pay for them.