Religious discrimination at school

My son is at High School. He has problems with his Physical Education teacher as she demands that he takes off his cross on the chain during the class. Otherwise she doesn’t let him participate in the class activities. We are Russian orthodox and always wear the cross. The teacher wants us to bring the later from our clergy saying that the cross should be worn. Muslim female students are Not asked to take their scarves off or to bring the letter from clergy. Is this considered a religious discrimination act? As to me- yes Thank you