The sister who graduated has also gotten a bit of press attention as she made a cameo in one of Lady Gaga's videos.
I disagree that she could have necessarily gone incognito, even if she wanted. Most seniors in high schools (especially a smaller private school like this) are pretty familiar with their classmates and even their classmates' families. And because Gaga is so popular with that age group, I think most every graduate in there knew that their classmate is Lady Gaga's little sister and even did a video with her. Even if Gaga dressed down, all it would have taken is for the kids who knew Natali's (the sister)'s parents, find them in the crowd, and then spot Lady Gaga with them, whatever she was wearing. She would be hard to hide, as even dressed down because Lady Gaga and her sister look uncannily alike, almost like twins.
At another event it might be a faux pas but at a high school graduation, I think the grads might have been MORE disappointed had Lady Gaga not shown up, or shown up in a boring outfit. Any high schooler I know would rather have seen Lady Gaga at their graduation dressed in her style rather than her not show up at all, or show up and "be boring" because she might offend the parents in the audience. Most graduations are a long bit of monotony with about 30 seconds of individualized excitement (when "your" grad walks across the stage). As long as made no ruckus in the audience to disrupt that, she would be fine.