Open it up to men. Keep it called “Women in STEM” and directly maintain in your mission statement that your club is aimed at promoting and supporting women in STEM and engaging a new generation of women in STEM. You are working to support an underrepresented population. Thats not a divisive thing no matter how some guys will view it. If they find a problem it’s coming from their own beliefs.

I’m a man in STEM, full disclosure - my mom was a scientist, my wife a microbiologist, my sister an engineer. If it existed in my backwoods school I’d have supported that club if not by membership (just because I’m not sure I’d be the target member demographic) then in donation or patronage to fundraisers or whatever. We seriously need more diversity and inclusion in the field. Some men are afraid of equality. Unfortunately, I suspect you’ll see that and may need to prepare to face challenges due to this. I’d like to think most men won’t be offended by the idea of women in STEM, though. More likely (sadly) they’ll just ignore a women’s group. Some will genuinely support the group’s cause and will be respectful and open to listen.

If you open the group rather than disbanding or taking it underground, be clever but fair without a doubt in your bylaws. Have a clause about expelling members who display behavior contrary to the goals of the group and make it well known that this is specifically a group aimed at supporting women. If challenged, bylaws that favor one demographic in club leadership or inclusion will be overruled by school administration. The entire mission of the club is to support women, and if men want to join honestly it might be good for other boys to see that people of quality don’t fear equality. Good luck! You rock.