Theatre, eh? I know a few of these.
Written In the Stars - AIDA
This one may be a little dramatic, but it's a good showcase vocally. A more contemporary vibe, also, might make it worth considering. I'd start from, "But nothing can be altered," if you're just doing a section of it.
The Word of Your Body - Spring Awakening
Another more contemporary piece that would be really good for what you're looking for...they sing a few lines each, then some unison. Pretty melody, too. Maybe a little risque connotation for high school, if that's what you're doing it for. But other applications would probably be excellent.
The Last Five Years - The Next Ten Minutes
All right, so granted, this starts with a male. But it's such a wonderful little tune. Very sad, and very good for a duet standalone piece; I've seen this done brilliantly before in showcases.
Wicked - As Long As You're Mine
Also seen this one done beautifully in showcases. Granted, it's not an exchanging lines sort of duet, but it does start with a female. And I feel like it's a really good song for a lot of different sort of singers.
Tick, Tick...Boom! - Green Green Dress
Okay, so this one's really risque. But it's a hell of a lot of fun, and I find it upsetting that I see less people doing this one. And I suppose it does start with the male vocal - but I mean. I dig it, and maybe you'd be able to do this one.
Miss Saigon - Sun and Moon
Such a pretty duet. Just...yes.
Sweeney Todd - A Little Priest
I included the version that isn't the crap film version. Such a good one. Funny, dark, and a lot of fun to do.
Phantom of the Opera - Phantom of the Opera
If either of you are classically trained, this one is especially effective. A lot of people won't touch this duet, but if done well, it's superb.