'This policy has been established for some time and follows procedures used by many schools and academies across the country.
However, Mrs Buckingham's daughter had lost the one allocated to her - and so believed it was impossible for her to leave.
'During breaks and lunchtime toilets are open and accessible to all students.
Rhian Lewis tweeted: "Stereophonics were amazing.
'No-one should hold their bladder because if you don't go to the toilet it can cause problems.
The only way they can go is by presenting a 'pass' which allows them a break to relieve themselves.