Dd1 would like to do football after school on a Monday (there is no netball/hockey for Year 3's). One other girl does it (there are about 15 boys) but her mother said that the head was very negative and in fact is watching the practice today to see if Katy 'gets near the ball' - if she doesn't she is going to stop her doing it.
My sister had this problem when she was in primary school, she wanted to play and the head wouldn't let her. In the end my father had to go in (as my mother's arguments fell on deaf ears) to talk to the head. She was then allowed to play and ended up being captain of the school's team! Keep perservering. Whether or not illegal I would say you could put together some pretty good arguments about sex discrimination.