Network effects are powerful recruiting tools. You should find a way to help the girls already in the club to recruit friends.

Another angle would be to remove your assumptions from this. Survey other girls who aren't participating and find out why they didn't join. Is it because they didn't hear about it? Other activities they're already doing? Disinterest in the subject?

I do suspect this question will ignite a cultural flame war on HN, as it is certainly possible that there are no gender barriers for that club other than genuine disinterest.

I'm a middle class white male, so idk much about these things. But my first idea was to put phrases like, "Did you know the first computer programmer was a woman? Ada Lovelace" and other surprising, encouraging facts about women, minorities, and inclusivity in STEM on posters, flyers, or whatever.

I think one of the keys to getting anyone (child, high schooler, adult, etc) interested in something is to make it cool. Making something cool requires a different approach for different groups. Adults might think it's cool to make a lot of money, but kids often don't care about that. But if you know anything about your child's generation and what her age groups thinks is cool, try to find facts that highlight how "cool" STEM is in their context.

Please don't interpret my use of "surprising" to be condescending, I mean that I think most high schoolers will find these things surprising.

They are fairly expensive but if You're working on a grant proposal You might want to consider asking for funds to purchase a CNC embroidery machine and CNC fabric cutter.

I would suggest a Bernina embroidery unit and a Brother scan N Cut. Tie this in with Arduino LilyPad Etextiles boards and I think you have something.

This could have a side benifit by encouraging more boys to engage in the textiles and fashion industry.

It has always seemed strange to me that girls are less interested in robotics, all the while using sewing machines which IMHO are the most complicated mechanical machines in a typical household, is embraced mostly by the female sex.

I tried to get my daughters into programming. They didn't want anything to do with it.