What Makes Them Think That?

I arranged my school books on my Dad's office desk and began to settle myself in to get crackin' on the homework while my Dad was busy. A colleague of my father's peeped in to introduce herself and offer me coffee. I had spent a month in Philadelphia, and today was my first day back in Pittsburgh. He must have mentioned this to her, because she asked me if I was glad to be back home, and so on and so forth.
"Do you go to school in Philly?" She asked me.
I smiled politely. "Oh, no, I was just visiting. I go to school in Pittsburgh."
she continued to make small talk. "What grade are you in?"
"Just a Freshman."
"Do you go to Carnegie-Mellon?" She asked, naming a local college.
".....in High School."
"OH!"