This is a tough one, and I see where you are coming from. It is hard to justify giving your kids rewards like vacations when they are not doing "their job" of doing well in school, which is what we ask ours to do.
We have always told our kids that even if we can afford extra things, you still have to have good grades and behavior to earn them. We haven't made them get a job in high school because they were involved in other activities, though they have earned extra spending money doing things for neighbors, etc.
We did a WDW trip during older DS's senior year as a celebration. Younger DS asked that for his senior trip we do it as a joint birthday present trip for him (he turns 18 about 2 weeks after graduation) and invite his girlfriend and his brother's girlfriend, who he considers like his big sister. If older DS was not doing well in school I would have no problem telling him that because we were having to shell out more for his schooling than we couldn't afford for him to go on the trip. But we are not planning to pay for the girl's food or tickets- just the rooms. Because older DS is in the Air Force, going to college, and paying his own way for everything, I am more inclined to want to pay for his part of the trip. His GF is also doing well in college and works, and I know her parents will chip in for her. Younger DS's girlfriend is also an excellent student and her parents will pay for her.
I guess my point is that regardless of if she comes in the trip, she is old enough for you to need to have a conversation about her responsibilities, both with grades and finances. That is most important. Then you can decide based on how she does this year if you pay for her. Maybe if you bring up her paying for part of it she will save her money and work a bit harder in school. Overall she needs to know that you want her to succeed in school and life in general so she will be happy, no matter if she goes on this trip or not.