[impendia]

Do you know how to be a guest speaker at high schools? Do you just cold call people?

[squasher]

Yes, you could do that. The department chair is probably your best bet. You could also network if you know any teachers. Are you in SF?

It's nice if you can build a relationship between your institution and a school so you can have a sort of standing gig every few years.

(Also note that some schools will totally love this and others might be unresponsive, so don't give up if the first one doesn't happen)