Nothing throws my Middle School classroom off more than "But I asked to go first!" disputes between students. I've put together a .pdf file that solves this problem.
Students sign up as they arrive to class and once it is their turn, they can head to the restroom without losing their place in line!
This does take a bit of reinforcement to establish this procedure in class, but it has worked quite well for me!
Take this project up a notch and make a tear-away notepad to hang on the board!
Or laminate and use with dry/wet erase markers!