This has probably been invented, since it's so simple, but I'll give it a try anyway. How about a flashlight that has a computer chip inside that flashes S.O.S.

I definitely like the metalic paint that is used on special made motorcycles. Though it cost about $500 a galon. A pen may cost only a few dollars, and on paper the ink would change colors in the light. Kids would love it. Teachers would hate it.