Question

If a friend not selected at school dance competition then what the reason she become rude towards their dance participant and teachers

Answer 1

because she is not selected and she will be very angry about that that's why she is rude towards their dance participant and teachers

Answer 2

She is very angry because  she wasn't able to participate in the competition. She is very sad as she alone is not able to represent the school at the group annual dance competition.She is feeling inferior to her fellow participants and she is angry with the dance teachers as they have not given chance to her.
           I would advise her not to feel sad. I would advise her to practice more dance steps so that she could get a chance next time.