“We lost touch with each other after high school.”
You've been sending Facebook messages to an old friend who you haven't spoken with in many years. Your wife asked who he was. This is how you explain your history with this friend.
We lost touch with each other after high school.
To "lose touch with" someone means to stop communicating with them. It's kind of the opposite of "keeping in touch with" someone.
When you "lose touch with" a person, you stop calling or e-mailing them, and you usually don't know what they've been doing recently.
To explain how long it's been since you contacted a person, use "for":
We lost touch with each other for a few years, but he called me last spring out of the blue and invited me out to lunch.
Elementary school, high school, college, etc. are places where you go to school, but they're also time periods in your life. So you can say "before college", "after junior high", and so on.