If you're looking to reach kids in public schools, then you need a 'third space' outside of the school environment where they can meet kids who are doing things the other way.
It's not easy for a kid to 'unschool' without family support, so it could make a big difference to just gently open their minds to see what other families are doing, and meet some wonderful kids who are doing things that way and turning out great.
I used to work with TrackersEARTH, which ran both homeschooling and school-break and after-school programs, and was a great place for kids to explore together while meeting others not from their school.
Other places that might sponsor activities where kids can meet up in a productive, self-guided-learning atmosphere include libraries, after-school clubs, scouting, 4-H, church groups, etc.
These activities have the plus that they are great college-prep or career-prep and feel very comfortable to parents, yet with the right adult leadership they can heavily feature self-guided learning activities, hands-on or service-based learning, etc.
If you are an upstanding citizen who can pass a background check, many such youth activity groups welcome adult volunteers who can add new skills or just a listening ear and mentoring perspective to these activities. You could offer to chaperone a fishing trip, teach a
gardening or food-foraging session, help with a merit badge, or whatever skills and opportunities are easiest for you to offer.
-Erica