How are you surprised that they did it when they knew what high school she went to? Two possibilities:
Option 1: Step 1) Find copies of yearbooks going back a few years, 2) find her face in the yearbook.
Option 2: Step 1) Search facebook for people who went to the same high school as she did in the probable time frame. 2) Look for her in that set, or in the set of friends that the people who matched the search have.
Feel free to sprinkle in face detection on the yearbook/facebook photos to make it more engineer-ey.
As a matter of fact, how did they find out what high school she went to? I can't imagine you telling them that if you were not providing much information about her to anyone else.