As teachers, we like to imagine we a modicum of control of our classroom environment. I have the relative luxury of "my own" classroom: I teach in one classroom and I have unfettered access to that room during my prep period. Not all teachers have this. I did not always have this.
When the school year starts up, you want to conveys some simple dos and don'ts. (Yep, I wrote "dos and don'ts". Because that's the correct way to write it.)
So I made a poster, borrowing from the old Highlights magazine cartoon, "Goofus and Gallant". I don't presume anyone gets the reference other than me. But one or two students might google it at some point, and that's good enough for me.
In any case, here's the first production of my poster. It's designed for print at 13" x 19".
Red is bad (Goofus); green is good (Gallant). Yellow lies somewhere between.
A nice physics bonus will emerge when we study colors and discover what red+green makes.