That doesn't mean she'd get a huge response from the hometown crowd who remember her before her fame. The mystery and glamour would not be the same for them because they'd remember the embarrassing and mundane moments from her life.
The problem with this is that it's not just the hometown, which actually happens to be Manhattan, that are at the graduation. The crowd is going to be mostly made up of people who did *not* know her, which would be friends and family. And many people come and go in high school, which I would think would be especially true of NYC. Her sister is also 6-7 years younger than her, so most of the people there would not remember her. She isn't going back 2 years later, where freshman and sophomores are now seniors.
I think people are underestimating the kind of response and distraction she would be. I read, last week, that she is the most popular living person in regards to social media (Twitter and FB), and is the first person to reach 10 million fans on FB. As someone who is a fan of hers and does enjoy her music, while I wouldn't be taking pictures or accosting her, I would definitely be interested in what she was doing, and would probably watch her throughout the graduation. It wouldn't be very hard to do, considering how much she would stick out like a sore thumb.