There were exchange student programs she might have planned to be a part of. Some students in my high school were sent to school in Switzerland for a year. She lived in Washington - maybe she used her passport to go to Canada (she didn't need one, a bc would do, however it might have been an excuse to get a passport).
My high school (way back then) sponsored summer trips out of the country for students (my mom wouldn't let me go).
A passport back then was only 10 bucks and a useful form of identification both in and out of the country.
So what is your theory about the missing records?