Have you considered advertising through high school gifted programs?
I would have benefited a lot from this type of mentoring in high school, and when I think of how you could have reached high school me, the most plausible path would have been through my school's gifted program. I grew up in a mid-sized city where I more or less exhausted the educational opportunities for high school students, and never attended academic summer programs or anything similar. I also didn't use the internet much, and I think it's unlikely that I would have stumbled across this on my own. In fact, by some of the same tokens, I might have been an unusually high-impact person to reach because I wasn't likely to get the type of information you are disseminating through other avenues.