This simple little acronym has been around for a while, though I've revised it a bit for IB purposes.
Before you speak: THINK
T Is it true?
H Is it helpful?
I Does it promote inquiry?
N Is it necessary?
K Is it kind?
I've found it's quite helpful to introduce this acronym at the beginning of the year and post it in a prominent place in the classroom, reminding students when they stray from the Think Before You Speak agreement. Sometimes I just have to point at the poster, a student will stop in mid-speech, and either steer their comment in a more positive direction or just say something to the effect of, "never mind." For students who have a challenging time with impulse control, it can be helpful for them to have a laminated copy on their desks as well. Useful on the playground? You bet! I haven't seen it used in secondary schools, but I bet it would come in handy there as well.
At home with your own kids? I haven't posted it yet, but I might just put it on the wall beside our dinner table. Too bad there isn't an M for manners in the word THINK!