Look up the bog standard campaign for more info. This is a campaign by ERIC (Education and Resources for Improving Childhood Continence) who are trying to get instant access to toilets for students. This is from their website (adults section)...
"For children to stay healthy, they need to drink water regularly throughout the day. They also need to empty their bladder and bowels regularly and fully when the need first arises."
"There is a widespread expectation that children should go to the toilet at set times irrespective of whether the child needs to, in order to minimise disruption to lessons. It is all too easy to reprimand the child who needs to go during lesson time with "You should have gone at break!" However, having set times for access to the toilet can cause ?I?ll go just in case? practices which means the bladder doesn?t get used to holding on until it?s full. Over time, the bladder capacity can reduce, increasing the need to visit the toilet more frequently. At the same time, the amount of fluid a child can drink before needing to go to the toilet is reduced. This results in a vicious circle. A child may consciously or unconsciously ration their fluid intake, or avoid drinking altogether, if they fear not being able to go to the toilet when they need to."
They're pretty clear about what they state to be best practice - free access to toilets. I'd print it off and show it to the school.
HTH