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

I have never come across any legal provisions defining rights of teacher to allow or not allow students o use rest room.

But I quite understand that teachers must show some flexibility when student in their class ask permission to use the rest room. At the same time we should also understand such practices cause disturbance in the class. Also students must learn to discipline themselves. If they don't, they will have serious problems in their working lives.

In  general teachers and schools adopt more permissive approach in lower standards, gradually the discipline is tightened in upper classes. I believe by the 12th grade students should be disciplined enough to not to use rest rooms during class hours barring rare exceptions.