For school aged and college students it is really that much different for the majority of students. I say this as having been a student taken out of school for a cruise. I was in high school it was just before winter break and I had to do a lot of arranging ahead of time. Sometimes school calendars also favor the Thursday/Friday for days students would be out of school like for parent/teacher conferences or professional days, school breaks, etc. That's not even getting into laws surrounding school attendance and then district rules when the state has less black/white rules.
I think most of us as adults who have worked jobs with PTO also can understand how the difference between the number of days we take off especially if you can't put it with a holiday which reduces the number of days you actually have to use.
That's just speaking about the PP's complaint regarding taking kids out of school for the whole week as opposed to later on in the week as there is a point towards how balancing out a schedule for selected itineraries can work; ultimately it won't be ideal for all especially with having so low of days you can sail.