I think we are confusing the word "justify" with the word "excuse." I don't really know our schools policy and frankly don't care. I have never taken my kids out of school for a vacation, but they do frequently (2-3 days, sometime partial days a year) miss for dance competitions. I have never told how that is "educational" even though it is and could, if ever asked.
However, we justify our taking our kids on vacation whether taking out of school or not every time...ie, where to go, how much money to spend, especially in a tightening economy, whose ideas to take into account and whose not, etc. But, by school standards, my understanding is that to be "excused" in some school systems, an absence has to be educational. That requires (IMHO) a list of ways it will be educational. If you assume that the principal/teacher will know how it's educational, they may not assume that you have thought through the educational aspects and may not present them to your child. I don't believe it is unreasonable for a parent, when asking for an absence to be excused, to explain that care will be taken to make the trip educaitonal. If it can be tied directly to class topics, all the better. For example, a trip to Boston, Washington, DC or Philadelphia might be more educational than a trip around the world, but only if the topic is US History not World History or World Economics. Last year, when we cruised on spring break, my son was studying the Mayan Ruins....wow, what a great opportunity for us to tie our trip to something he was studying.
In an aside, but hoping that this is still on topic as the topic was a letter that others are stating isn't a necessity and I am trying to make a case that it is....When my son was in 5th grade, a student in his class accompanied her college professor father on a tour of different countries. It had to do with his job, but I don't remember how. She was gone for a full month.
When they came back, she gave various reports to the class on her experiences, including a slide show of some of the countries they visited. What a great educational opportunity her trip gave not only to her, but to all of the students in the class!
Please think, when you take your kids on a trip, whether in the summer or during school year, how your children learn and grow from the trip. Take the opportunities as they present themselves to help your child learn about the world we live in and how to be a responsible, productive member of society. I hope your children are excused from school, are able to make up their work and bring back lots of experiences to share with their classmates, so that they, too, can share in the educational opportunities you afforded your child.
Some thoughts:
We bring back a newspaper and foreign currency from each port. Now that both kids are taking Spanish, they always get extra points for stuff from Mexican ports.
If there is something that ties to a recent lesson, we might pick up a post card, pamphlet or something to take in to show the teacher, possibly even share with the class.
If they meet people from different cultures, they are encouraged to ask for information regarding their culture. If they have a different alphabet (ie, Chinese and Arabic), and seem willing, we ask them to write something for us, often the kids' names.
Writing a report about the experiences or keeping a daily journal, helps not only with the historical/educational aspects of the trip, but it ties into writing as well. In this way, you are almost always able to tie in to English class. If your child's class does daily writing topics, maybe you can get those in advance.
You could have your child budget the entire trip to find out how much it costs and figure out some formula (ie, cost per day, cost per meal, compare it with a land vacation, compare with staying at home and eating out, etc.) that will tie it to math.
Social Studies seems to be a no brainer. But if your child is studying social groups (mine did that in 2nd grade, I think), then find out about how they work in the cities you are visiting. BTW, the ship has lots of social groups, find out how they work and tie it in, if you can.
While I haven't written the letter for you, I hope you can take some of my random offerings and put them in one of the letters you have already written.
Thanks for reading this far. HTH.
I remember booking this cruise when Matthew was 4 and telling him he had to wait until he was 6, when his bday hit in June we were both excited, closer to the cruise
i will say we are going on a cruise so i dont have to call him out while im on the ship but for our days in disney afterwards he got sick when we got home i already told my dr i need a note just get n good with yr dr all i gotta do is buy him a drink
From your lips to the big guy's ears! 
