I am also in highschool and this sounds very similiar to a situation I am in with one of the girls i have been getting with.
Last week she couldn't get enough of me and now she is showing no interest and intentionally trying to show me that she can have another guy by cutting me off in the hall to grab onto some other guy, I respond by being unaffected just like skittlepimp said, never chase them!
All she is trying to do is get an upperhand in the situation and when she sees that your unaffected she will hopefully want to come crawling back

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Good luck, and just keep gaming other girls while your waiting to see what happens with this one girl. don't let it affect you to much.