I feel you bro. I'm in high school btw. Plenty of girls that I know do sh1t like this.
They'll look at me and I'll catch them on multiple occassions, but I don't know them so I don't say anything (I don't do approaches).
I feel like girls who do this like you. You ignored them in the past, or they felt ignored by you, and now they've given up trying to show their interest in you, so now they just PURPOSELY try to ignore you. They ignore you intentionally, but they most likely are interested in you secretly.
It'd make no sense for a girl that isn't interested in you to intentionally ignore you. Plus, a girl who's interested (and is normal, and shows it) in you obviously wouldn't ignore you...
Whether she likes you or not, there's one thing I can confirm: You've messed with her emotionally to the point where she intentionally ignores you. You've got her going out of her way just to ignore her. This means you matter enough to a certain degree.
There's this girl right now that used to come up to me ALL the time, used to call out to me and talk to me in school. I didn't really see her as attractive, so I never really saw her sexually/romantically; although, I did give her attention back. I would at times conversate and reciprocate the attention that she gave me.
Then, out of nowhere she just begins to ignore me as if I've done something (and I havent, lmao). She doesn't even look my way even if I'm right in front of her, almost as if she purposely tries to look away from me. And then when I WILL try to conversate with her, she'll be very unresponsive... she won't get very lively as she used to be with me; and depending on the situation it'll seem like she's coming off as cold and distant. One time she even gave me attitude.
I'm not going to over think it, and I don't think you should either.
I think the reason for this behavior is just simply put like this: The girl used to like you; you didn't give ENOUGH attention, so now she's mad/sad whatever and for whatever reason, she goes out of her way to intentionally ignore you.
In my case, I actually tried to talk to this girl and she continues to act cold towards me, even though before she started to behave like this, she seemed like she was interested in me.
In your case, you haven't talked to her yet, right? Why not try talking to her? There's nothing to lose here, man. Suppose she does actually like you. After you finally give her some attention and talk to her, maybe she'll stop acting so dumb.