A teacher can deny access when she has reason to believe that the child is making things up. More information is needed as to your child's history with this teacher, and why the teacher did not take him seriously. Depending on how old your child is, there should be plenty of witnesses to what happened.
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Teachers have a great deal of latitude and discretion in managing their classroom students. Your child's teacher can deny them the privilege of leaving the classroom; however, that may be modified if you have verified a bona fide health condition. If your child has a health condition that requires access to a nurse or frequent restroom use you should provide that information to the school nurse or principal, that will facilitate your child's access in the future. Teachers make thousands of decisions every day for the many children they interact with, sometimes the get it wrong--looks like it here. Talk to the teacher. If you are not satisfied with the answer, talk to the principal. Before I was an attorney I was a school principal, I always told parents, "I don't believe everything your child tells me about you, please don't believe everything they tell you about me or the teachers."
Hope everything works out for you.
All the best!
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