The answer to the core of your question is pretty strait forward....girls must be provided with equal opportunities to compete in a comperable number of sports. In other words, if there are 100 "positions" availabe on boys sports teams then there should be roughly 100 "positions" available for girls to compete in organized sports. The compostion of sports will be different as girls do not play football, so other sports like vollyball or soccer should be establish to insure an equal opportunity to compete in sports is available to all students, regardless of gender.
In a situation where a baseball team exists but there is no softball program (a classic comparison) the school must do whatever is necessary to provide an opportunity for girls to play softball.
This situation is very likely a violation of Title IX, and in my opinion should be taken to the school district's Title IX Compliance Officer (federal law requires they have one and that they perform an annual survey and report any deficiencies to the school board) for resolution. If they fail to resolve the issue to your satisfaction, get in touch with the Office of Civil Rights, U S Department of Education in Atlanta.