Gretel,
This is a tough decision and there are various factors involved. This answer is coming from me as a parent and high school teacher:
-your school's attendance policy (does it affect grades or credit)
-the willingness of the teachers to prepare work in advance
-the willingness of the teachers to work with your child upon his or her return to "make-up" the work or additional instructional time after school. (I don't want to open a can of worms here, many teachers are very willing to work with a child after school however please don't expect the teachers to spend hours with your child providing individualized instruction due to a vacation.)
-the availability of tutors to help your child catch up (National Honor Society members often tutor for free as part of service credit, other tutors are available for a fee)
-your child's ability to self motivate and self study - given the material, can you child work through the material on his or her own and produce the required products? Even if there are not formal assignments to produce and the assignment is simply "read pages 10-25", obviously classroom discussion and other activities are related to and involved with that reading. As a parent, you may have to be involved in your child's assignment, talk about the reading, etc. to help your child understand.
-time and dedication necessary to play catch-up post cruise and stay on top of current school material
-parent willingness to support the child to catch-up - the parent shouldn't do the work for the child, but should certainly be supportive in helping the catch-up process. Please don't abandon the child to work on this on his or her own - catching up can be an overwhelming process for some students.
If I were to generalize and simplify, it is "easier" to miss elementary school because often the teacher can send along the worksheets, the tasks tend not to take as much time to complete, and parents are familiar with the material, enabling them to help their children stay on top of things. However in middle school and high school, the complexity of assignments and level of skills involved become deeper, and the result is it is more difficult to miss class and play catch-up. Not all parents have advanced math skills, deep knowledge of biology, or are fluent in French. Miss those classes for an extended period and the result can be cademic struggle.
Personally, I won't take my kids out of school for the 7 day cruise now that we are hitting middle school in our house. The stress of catch-up is not a stress I want to add to our life. So we choose to travel during spring breaks or holiday breaks - that way it truly is a family vacation.
Perhaps you can time your cruise to hit Thanksgiving week or other holidays that could minimize the number of consecutive days your children will miss school.
Again, this is a complex decision, and it depends on your child, you as a parent, your child's teachers and your school! As a teacher, I've seen some students go on vacation and be fine. I've seen others go and subsequently stuggle the rest of the year. You and your family are in the best position to make your decision!