Out of interest, what part of the country is this in, roughly (just a county/city will do!)?
I have experience of working with students of approximately that age (i.e. 11 I presume) in North Yorkshire, and I've
never heard that phrase. I had to look it up!
You may want to read this thread:
Were toilet breaks during lessons allowed at your school
The best thing to do is comply with your employer's policy; I would also seek clarification about what the policy is if you think someone has an exceptional reason. Someone I know who is a teacher said his policy was to allow students to go providing they agreed to stay back at the end of the lesson to catch up any work. This had the effect of reducing the number of requests drastically. It doesn't work if the students immediately have to go to a different lesson in another classroom at the end of the lesson, but works well just before break/lunch/after school.
And if they use inappropriate language and/or are rude, you can issue whatever sanction you deem fit, in line with the school's policy.
I would expect most schools to have a toilet pass system for any student who has a particular need and I would anticipate this would likely be arranged by their Year Leader.
I would recommend speaking with a colleague at the same school if possible, or alternatively discuss it with a friend who also works in a school.