my child is 6 years old and she is humping her school chair

I don't even know how to talk about this my child is 6 years old and she is humping her school chair that is the only time I see her doing this she will do it infront of the whole class just recently she had her awards ceremony and sat and did it the whole time I have tried to talk to her about it I don't know what to do. I worry and I am embarrassed I don't know what people think please any comments or advice I would really appreciate