we did a 14-day Celebrity Mediterranean cruise with our middle schooler. It was November and the Thanksgiving break mitigated the time out of school.
We worked very closely (and very early) with all of her teachers. She did all of her math lessons on the plane on the way over (thank goodness she is great in Math, because I was looking over her shoulder and was mystified). Her social studies class was doing a unit on ancient cultures and was scheduled to do Greece -- her teacher gave her a pass. She had to do a report on volcanoes for science. No problems reading her book for English for a few hours one at-sea day when it was a bit too chilly and rough to do much of anything with her friends. We had two stops in France and started in Spain, so her foreign language teachers also gave her a pass (she brought back some treats with Spanish and with French labels as a bonus). She is really very good at managing her time, and is a good student so the teachers knew she wouldn't be just sitting by the pool.
It was a great experience (Spain, France, Monaco, Italy, Croatia, Greece, and Turkey). She met a bunch of girls her age -- an American girl whose parents worked at the embassy in London, another whose mother was stationed with NATO at The Hauge, and sisters whose father worked for the Italian office of a US manufacturer on the first leg of our trip. And then made friends with a brother and sister from Portugal in the second leg.