I haven't read all of the posts so I apologize ahead of time if I'm repeating what others have said
15 days unexcused allowed? That is a LOT.
Think of it this way - who here would LOVE to have a job where they got the summer off, extended time off during holidays, Other days off randomly throughout the year for non holidays, no required nights or weekends unless you do something extra that you WANT to and THEN they tell you you can be absent for 15 days without a good reason without repercussion above sick time?
Anyone else ready to line up for this life??
Flame me if you want because I have no kids but I did play a kid in a former life about 20 yrs ago

- but - I was not pulled out of school on a yearly basis for vacations. Who am I kidding, my mom and I could barely afford food so vacations were few and far between but even then - trips to see family were done school breaks.
I will admit, in my sophmore year of high school I did miss 3 days of school to go to WDW. My brother offered to pay for me and my mom to go with him and his family during spring break and since the break in NY was slightly different than VA, we did overlap by 3 days. But that was it during my entire school career.
After reading some of the attitudes here with parents feeling their kids should be able to take a vacation whenever they wanted to something has been made crystal clear to me.
Now I know where these employees that I find that don't last but a couple of months come from. You know, the ones that DEMAND when they will take off, even though the company doesn't allow time off during that period? Then they find themselves with a permanent vacation?

I guess they had a lifetime of their parents allowing them to do what they please whenever they please no matter what the guidelines are with their "job" - whether it be grade school, high school or a real job.
Think of what message you are sending to your kids. You are telling them that it is OK for THEM to deviate from set guidelines. That guidelines and rules are meant for other people. Like I said I am not a parent but even I can realize you are doing such a disservice to your child by enforcing a "rules don't apply to me" mentality on them.
To the OP going on the cruise and having difficulty scheduling vacation time - does your company have that many full time people with vacation hours to schedule that it's impossible for you to pick a week that your child isn't in school? If the calender is that full of time already taken by senior employees, maybe you should talk to your boss and explain to them that it's to the point where vacations are going to have to overlap because of the sheer number of weeks that need to be scheduled. Have you tried to talk to a coworker that has a week scheduled during a school break that maybe doesn't have school aged children about trading weeks? I know you are between a rock and a hard place right now but if the teachers are adamant about your child receiving an unexcused absence for taking those days for a cruise - I'd cancel the cruise and rethink vacation schedules for the future years. You never know what emergency could happen later in the year (God forbid) that could cause you to need to take him/her out of school longer which would mean penalties to you.