Girl gives me mixed signals, help?

ok so she gives me all the right signals, smiling, saying hi, staring and then looking away. but once she was talking to this one guy, we were passing each other in the hall and i looked over at her. she looked at me and waved at me. i waved back, she loookedaway and smiled. 2 months later i asked her to a school dance and she said she already had a date but would have gone with me. she didn't have a date. but at the dance she kept looking at me. now al the signs have stopped excpet for the staring. what does this mean?