I also use for classroom signals. The first signs they learn are "bathroom" and "water".
The students may use these signs only when we are in the middle of a lesson and it is very important... I respond with sign language "Yes" or "No." Works well.
When we are in the classroom working independently, the students use a 2 pass system. Each student gets a number at the beginning of the year. I laminate 2 sentence strips and put one of the fram on either side of the door. It coordinates with my class theme, bees, frogs, dogs... At the top lives the "bee in the hive" or the "frog on the lily pad." and you simply put the bee on your number and move it back when you return. I can always tell who is out... and the students always know if they are free to go or not. Love it.