Girl Keeps Staring At Me In Class (High School)

I don't know if this is the right place to post this, or even the right forum tbh.

What you're about to read probably sounds really stupid, but I need some advise.

So there is this girl, we'll call her Molly. Molly used to go out with one of my friends, we'll call him Jimmy. I wouldn't say I was close friends with Jimmy nor Molly, but we had two classes at the end of the day last year and sat by each other, so we were good school friends. I kinda felt like the odd man in the group because it was usually the three of us and maybe another person whom we talked with all the time in those classes, but I never led a conversation or anything; I was just there. Molly also seemed to ignore me, not in a mean way, but in a disinterested way. We weren't really friends and whenever we did talk, I would usually start the conversation (not in a flirtatious way).

So she was like that the whole time during the previous school year. Then, one day early in the summer, one of my friends and I (we'll call him Tommy) walked into an ice cream place where she worked (I didn't know she worked there), and when she saw me, her face just lit up. And then while Tommy and I were making our orders, I saw out of the corner of my eye Molly said something to a friend and they started giggling. Then, Molly asked me later what I got on one of our exams, and I just said that I didn't know, and then she asked some other stuff to which I gave simple responses to. Tommy and I left and neither of us really thought anything of it, I honestly forgot that encounter even happened until recently. I didn't think she had a crush on me or anything. She was dating Jimmy for several months so I thought maybe she was just excited. Actually, I thought it was because of another girl whom we'll call Mary, who was in one of our classes that seemed to like me and was friends with Molly. Maybe Mary told her she liked me. I doubt it though because Molly didn't do anything at school to hook us up or get us closer, nor did Molly even seem to have any idea about it.

Anyway, a few weeks after the ice cream shop incident, A few friends (including Tommy) and I sent a stupid message to Mary over facebook one night while we were high. We wanted to invite a few girls to this one place for a party and at the time, we thought the message sounded genius, but it was actually REALLY bad lol. Anyway, Mary never responded and tries to avoid me at school. Later on, Tommy and I were at a movie theater and I saw Molly there with a few friends (no Mary), and I noticed that Molly knew I was there. She didn't directly glance at me or anything, but the way she looked at me, I had the feeling that she probably knew about the Mary fiasco.

The next time I saw Molly was the first day of school. We had two classes together, and we sat next to each other in one class. I didn't say anything to her at all, and neither did she. I didn't feel a tension though. I just didn't mind her at all, not in a rude way either. But then, I saw her one time in that class whisper to a friend while looking at me and they were smiling. I thought she was telling her about the Mary incident. Both of them dropped the class shortly after.

In the other class I have with Molly, I sit with a large group of male and female friends and we always laugh and talk, especially me. Molly sits with a girl and they only talk to each other. However, I had noticed over the past month that she glances at me frequently. I don't know how long she has been doing it either, I only noticed a while ago and now It's driving me crazy because she's doing all the time. It's just like an open-mouth stare, and even when she sees me looking at her, she's still just looking at me, no smile or anger or anything, just looking at me. It's really weird.

Sorry for the long story. Here's my question: why is she staring at me? I never talk to her at school, nor made an effort, neither has she. At first, I thought she kept looking at me because of how social I am around the big group of people, maybe she didn't know I was popular. I don't really think she liked me either because of her and Jimmy. But I think she may have broken up with Jimmy and even dated another guy I know semi-well (Don't know if they're still together, if they even were together). I also have no idea what people have heard of the Mary story, so maybe that has something to do with it, don't know what though.

I asked a buddy about this yesterday, and he thinks that she might like me, but that just seems weird to me. She never showed any signs nor anything. That meeting at the ice cream shop was just really weird now that I think back to it, because of how she reacted. It happened so early in the summer, when she was still pretty serious with Jimmy, but maybe she wasn't. I don't know, it's just kinda weird. Maybe it's nothing, and I'm just making stuff up because I'm some sort of psychopath or something.