A: You don't have to. So much time has elapsed, chances are there won't be enough twigs of shared experience to rekindle anything. Tell her in a cheerful response what a pleasant surprise it was to hear from her, give her the latest ages of your kids, and wish her well.
Q: One of my friends from junior high just tracked me down on Facebook. I don't have time to rekindle long-dormant friendships. How do I respectfully communicate this?