I would like to know if its allowed for girls to participate in sports activities, if she does it in full hijab.

In the United States I have seen girls participate in a sport in full hijab. It did require a waiver because the headress can become a safety issue. So depending on the level of the sport and what the sport actually is then it may or may not be allowed.

For the specific religiuos aspect you will have to start with your local authority.