
I issue every student in my classes a 16″ cardboard ruler. It stores easily in the 12×18″ folders where students keep their drawings-in-progress. Having a longer straight-edge comes in handy but the drawback is that, being made of cardboard, one can’t use markers or felt pens with it.

I had the rulers custom printed with my class rules and a color wheel on them. It cost around $80 for 1000 rulers. That was a few years ago and I still have enough to get me through the 2015-2016 school year before I would need to order more.
