How can I ask this girl to prom subtly?

When i was a senior...a few months ago...I asked a girl who was a sopre at the time, to my prom. We had a very good time..even though im a college freshmen this year, and shes a junior in high school. I know that her junior prom is in april or may and I would like to go with her...should i ask her and if so, when? we both went to different high schools...at my prom she didnt know anybody and hung around me the whole time...and i dont know anybody at her school. How can I subtly imply that I wanna go with her without being like..."Can I go with you to prom"? Like suggest it in a conversation or something?