I love the "done" sign. I have so many kids who want to get out of their seat to say, "I'm done!" When they do that, I come back with the worst dad joke, "Hi done, I'm Mrs. T." I will definitely teach them the sign language.
It really is helpful! It also lets me know who might need help first. I can wave for students who need to go to the bathroom from across the room and let students who are done know that I will be with them after helping someone with a question.
