What should one do in this situation?

I'm asking this question here, because hopefully I can get some input from moms and other teenage girls. Okay, so there's this girl I go to school with. There are 7 lessons a day, and I have lessons 1, 2, 5, 6, and 7 with her. I wouldn't say that we're friends, she's way higher up on the social totem pole than I am, but we're amicable and chat from time to time, and we have a few mutual friends. I noticed in lesson one that she had gotten her period. Luckily, she was wearing black leggings with a long shirt, but she left a streak of blood on her chair. That's just another part of high school- occasionally you'll walk into class and there'll be blood on the chair. It is really gross, and of course no one will sit in the seat, but I really feel sorry for whoever left it, because it's so humiliating. All of lesson two I was torn, trying to decide whether or not to tell her (I sit behind her in that class, so I could have discreetly mentioned it...). It seemed like she hadn't realized it yet, but I was worried that if she had noticed, I would have made things worse, because I think it would be really upsetting to have some girl you barely know remind you of something really embarrassing, and validate the fear that other people have noticed. And there was a substantial amount of blood on the chairs in lessons one and two, so I felt that one of her closer friends in one of the classes must have noticed and said something. I really felt like I should have said something, but social interaction, especially when involving an awkward subject, is not exactly my fort�. I didn't see her again until fifth period, roughly three hours later, and while she was still leaving a little blood on the seat, it wasn't as bad as this morning, and she was sitting on the very edge of her chair and kind of fidgeting, so I think she had noticed by then. What should I have done? Did I do the right thing, or should I have said something? If it had been a close friend, I would have said something, but... I don't know. What should I do if something similar happens again? I feel really bad, but at the same time I think I would have felt bad and awkward and embarrassed myself if I had told her. Help!