I heard that it is illegal for a teacher to not allow their student to use the restroom. Is this true?

While there
is no actual law that I know of specifically stating that it is illegal for a teacher to prevent
a student from using the bathroom, it is not that straightforward. A teacher, and the school in
general, serves the roll of in loco parents. That is just a fancy way of
saying that, when you are at school, your teachers are legally responsible for you the same way
a parent would be at home.

This all means that if there were a safety or
health reason that a student would need to go to the bathroom, your teacher would be legally at
fault for preventing it. This does not mean that you can simply go to the bathroom whenever you
like. It is still up to your teacher's judgement to decide if your bathroom trip is
necessary.

I suppose that a note from a doctor stating that a student has a
medical condition that requires frequent or regular bathroom breaks would legally enable a
student to have unfettered access to the facilities. Other than that, it's still the teacher's
call.