if you could get familiar with her social group, that'd be great. See ideally you'd want to show value to her group of friends without allowing her to be the center of your attention at first. Any chance you get to open her group of friends (including her being there of course) take it. This is when negs would come in great, directed at the girl you want to go to prom with. So while you're paying more attention to her other female friends or bonding with the male ones, she'll want some attention too, and when she does, throw a neg at her. Repeat this as many times as neccessary. Watch for changes in her behavior whenever you see her from that point on, keep an eye out for more ioi's coming your way.
Also, don't ask her after school. Be an adult about this situation, take her on a date, or at the very least get some type of a group hangout thing going on and separate her from the group during the evening. Always be sure to remain the value giver. For example, don't ask "would you maybe go to prom with me, please?" Be dominant. Say a statement instead of a question to get her to ask "are you asking me to prom?" You see how that works? Value givers don't ask questions, generally speaking of course