My first inclination is GO! And go with her!
Then you said it is a month long and you have other kiddos to take care of. There is no way in this world I would let my 15 year old go to Europe for a month. No way.
Two stories to shape where my perspective is coming from:
I was chosen for a similar honor in high school. I would have been 17 and traveling with an established group (including my beloved, and strict, teacher) for 2 weeks. I was very mature. My parents wouldn’t let me. I regret it to this day.
On the other hand, I was allowed to go to summer camp every year for two weeks. It was only a few hours away, but overnights. I, keep in mind, was always the perfect child. The camp was well supervised. At least two, usually four-six, counselors present at all times. And they were on top of things, too, not the lazy type. There was drinking, sex, and goodness knows what else going on. This is from the same age as your daughter is. If those kids could get in so much trouble (and I, the perfect child, didn’t do anything so bad, but I got in my fair share of trouble around them), then think how much trouble a month away from home, with supervisors who do not share your same values, will be.
Having been in your daughter’s shoes and truly understanding the desire, there is no way on God’s green earth I would let her go. Start planning a high school graduation trip for the whole family (or depending on finances and ages, for you and daughter or you, husband, and daughter, or you, sister, and kids, or some such). Then you’ll have time to save (put it in a separate account starting now) and time for your daughter to go, and time to spend with her as well.