It sounds like she didn’t want to go. There’s no way in hell I would miss a deadline as a scattered (non-ADD/ADHD) 16 year old for a trip that I wanted to go on. Plus it wouldn’t be a thing that she just knew about - every kid and the teachers involved would be chatting it up.
It does sound like the OP may be more excited about the trip, and hence, why the kid is taking the passive-aggressive approach of not following through so she’ll miss the trip.
I’d ask, straight up - “Do you really want to go on this trip? It’s okay if you don’t.”
Exotic travel is talked up as being the be-all end-all for young people, but some kids are scared, uninterested, would rather spend the summer in their hometown, etc. And what’s wrong with that? If the kid is reasonably intelligent and college bound, there will be opportunities for overseas travel later.
The grades are also a red flag. There is some organization and maturity that would be beneficial for a trip overseas. I certainly did the same bullshit as a teen - very smart but halfassed homework, assignments, etc. So I could totally be the one with a mid-C that would turn into an A after a lecture from a concerned teacher offering make-up credit. College disabused me of this approach real quick.
Here’s what I would do. I would have a calm discussion with the teacher about the kid’s performance, and ascertain what kinds of discussions ensued in class about the trip. It’s possible it was hush-hush so as not to rub it in for kids who couldn’t go. If it turns out that it was discussed quite a bit, that’s a pretty strong signal that the kid intentionally ignored the trip deadline.
Props to the OP for figuring out that there’s probably a little bit of “you should really do this!” going on. I don’t think sending her as if it was just a slip of the mind is sufficient. I agree with others that she should have to make up the difference of the late fee if she does indeed want to go - after being assured that it’s okay not to.
There could be any number of reasons why it’s not an appealing trip - lack of friends, jerkish classmates going, a special friend who is staying in town this summer, being scared of flying or being overseas, not liking staying in hotels… it goes on. I’d see if any of these issues are at play.