I have found non-verbal communication in the classroom to be somewhat ineffective. 

On the one hand, it often does not get attention like verbal communications.  In the case of the “quiet” symbol, it frequently takes a while for all the students to notice the symbol is “up” and quiet down.  The noisier the student is being, the longer it generally takes to recognize the symbol, unfortunately. 

This leads to the second issue.  If the point of non-verbal cues is to provide information in a non-intrusive, quiet way, these symbols need to operate in silence.  This is how it works in the military.  On patrol, the only language is the silent world of non-verbal communications through commonly understood symbol language.  Unfortunately, in schools, this is not the case. 

Frequently, the “silence” symbol is accompanied by “shhh!” from the teacher and/or other students.  More perversely, the “I want a drink” and “I want to go to the bathroom” symbols more often than not provokes a whole conversation between the student and the teacher.  “Do you have to go now?” or “No, you have to wait, you should have gotten a drink at recess.”  Both the silence and the class focus are broken.