Why would a girl stare at me like this?

Okay, I went to talk to the teacher about something and he was helping a girl and she is actually really pretty. So I was waiting infront not interrupting what he was helping her with. So this girl looked at me and looked back at her paper and then she looked up at me and was staring at me for a long time I stared back into her eyes but I looked away cause I asked the teacher something and she was still staring at me was this a dirty look or something?