*watches the scene where Hiiragi and Bell team up
Huh, you're right. It seems like Hiiragi and Bell might have met before. Of course, its possible that they'd met on the battlefield before. When they fought, they didn't talk any more than you'd expect two opponents to talk in an anime. They don't really act like you'd expect two former friends to act around each other, and Bell says that it wasn't long ago that they became friends. (meaning when they started working together, but the way the conversation was worded...)
Also, that promotional art is the only time that I've seen her in the school uniform. Its not odd for promotional art to do stuff like that just for the purpose of cosplaying. It doesn't necessarily mean anything.