California/ PE teacher saying "shut your trap " to her 7th grade student, while he was explaining to her that

PE teacher banned my son and his group from participating in a game....(2 games types, ans student my devide themselves into groups, and they were only allowed to chose one game out of 2. my son's group chose the next comming game...while the first game was going on, the teacher yelled @ the group for not participating..my son was ryin gto explain that his group will be playing in the next game, before she allowes him to explain himself, she told him:"shut his trap, and go sit aside" when i emailed the teacher asking her for explanation, and telling her that my son felt that he was treated ufairly, she denied saying that phtase, and she said that she told him to stop while he was talking not "shut your trap" and she told that he was the only one arguing with her. My son told me that he was only explaining because he was sure that it was unfair to put him aside..Please if you can explain for me how to handle this issue, am tired of her unfairness&rudeness 2 my son. do i need witness?