The SWE K-12 outreach (http://aspire.swe.org/) is probably close to what you're thinking of, though if you meant more of a peer-to-peer "dial a mentor" service I don't think I've seen one. Not that it couldn't exist.

Some other great organizations helping set examples and mentor high schoolers:

She++: http://sheplusplus.stanford.edu/

MentorNet: http://www.mentornet.net/

MentorSET: http://www.mentorset.org.uk/

Technovation: http://iridescentlearning.org/programs/technovation-challeng...

MIT: http://swe.mit.edu/outreach/

There are also several great organizations for women working in computing, who you could reach out to for mentors. PyLadies, DevChix, Systers, Anita Borg, etc... And local tech companies, at Google there were several folks working with local high schools who would put out internal calls for people to go and speak about the cool stuff they worked on.

Speaking of which, Google's Women Techmakers is also shaping up to be a great collection of women doing cool stuff in tech: https://developers.google.com/women-techmakers and https://plus.google.com/communities/100202454944694552166