Dance teacher shamed my sister?

My little sister is 9 and attends dance classes, last week they were discussing their routine and the competition they were attending and my sister desperately needed to use the bathroom so she asked and her teacher told her no, she asked again and was told no and it got to the point that she was in pain so she her friend and her just got up and went. When they came back out the teacher shamed her in front of the entire class and made a huge deal and just basically over all humiliated her. There have been other instances where she’s sent them off into groups of three and told them to create a little group routine and every time she would give each group a candy treat except for my sisters group. What do I do? She wants to keep going but I know it hurts her.
Dance teacher shamed my sister?
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