Confused when approaching a girl today

I'm only 16 and in high school. So there was this girl today that was in my class and I remember her from a class I had freshmen year. I'm a junior now. I caught her looking at me a few times and the first day I walked into the class she looked at me looked away and smiled. I thought this was a good reason to approach but I waited a few days. I approached today but made sure I looked at her a few times prior. I went up to her and said I remember her from that freshmen class. She said she doesn't remember me. This makes no sense because I was basically the center of the class and was always talking. Maybe it's because I look different now? I don't know. I said I was near the middle of that class and she said that's probably why she didn't remember me. I was going to continue to talk to her but she seemed to start walking away. I figured she wasn't interested so I just walked off. I'm not mad or anything as it's her loss but I'm just confused. Did I just read those choosing signals wrong? Was she nervous? Were they even choosing signals at all? I don't really care if she's not interested I'm just really confused. Maybe she is playing games? If so I'm not playing into that bullshit. Anyone who is experienced that could help? At this point I'm just done with approaching women. I'm not even bad looking. I take care of myself. I think I just may need to continue to focus on myself. Approaching just gives the other person the upper hand I feel like. I dont know any advice would be appreciated.