Print out the red and green signs, put them back to back and then laminate it. Punch a hole through the top and hang it on a hook at the level of your students. Flip it to green when it's ok for the students to go to the restroom (like during their independent time). Flip it to red when they can't go (like whole group time). Hang the boys and girls pass at the level of your students so that they can reach them.