I agree with Seventh Crow and I also have another thought.
Sex in general can also be a very new and unexplored subject in high school, especially the lower grades. At that point in someone's life, there's a good chance that they aren't completely comfortable with sex IN GENERAL, let alone sex with multiple people.
This tends to be a little more difficult for young women as well because they tend to be a little more emotional or attached when it comes to sex with someone.
This may or may not be the case with her, but its something to think about. She may just need to become more comfortable with sex in general before she's comfortable sharing that part of herself and/or her lover with someone else.
As always, use safe sex practices. You don't want to be a daddy yet, and its a very bad thing to infect your partner with an STD you caught from another partner.