Em Holliday

Each child has their name on a laminated card. When they go to the loo they stick their pass on the "Bathroom pass" sheet (velcro is best). When they return the name tag goes back on the desk. Through this you can limit the number of children gone from the classroom at any one time.

One teacher I saw had three ceramic animals which the student placed on their desk when they were gone from room. It worked well as you could clearly see who was out.

Have a sign in/out book for the toilet. This will also help you monitor how often particular students are leaving the room and whether it is always during maths!