A lot of people have already touched upon several good points. There are very subtle ways that girls can be excluded from team activities, and it's even harder to point these things out as they're unnoticeable unless you're the one being targeted. There are some excellent programs out there that are doing amazing things to provide better representation for women and minorities, but in general, the STEM community needs to change.
The STEM community can be horribly exclusive. There's this overall impression that you have to be this natural genius to participate. I'm sure you've come across these people at one point in your life, but there are a couple of elitist gatekeepers in STEM that just enforce that stereotype, and take it upon themselves to weed out people that they consider not to be up to par. Which is really silly, because there shouldn't be an entry-level for appreciating STEM. Advances in STEM are meant to benefit everyone, so even the most basic-level of understanding deserves to be validated and encouraged.
To fight sexism the STEM community, we can’t only advocate for the girls who already excel. We also need to actively support and engage the girls who want to participate, but aren’t conventionally good at math and sciences, even if they don’t end up being STEM majors.
tl;dr
http://goo.gl/mnBGvP