"Est-ce que je peux aller aux toilettes?" is asked over and over again in the French classroom. Simple print copies of this sign out sheet and leave them on a clipboard by your door to help facilitate easy sign outs and sign ins.
Keep track of who goes often and know who is out of your classroom in case of an emergency.
Bonus: "Est-ce que je peux aller aux toilettes?" is printed at the top of the signout sheet so if students forget how to ask, you can simply point to the sheet instead of frequently repeating yourself.