I've been taking my kids out of school for one week each year for a family vacation against all the district rules

. Fortunately, most of the teachers have assigned work and extra projects for substitution of what they'd miss but there have been a few that refused.
I agree with gopack that once they are older it is MUCH more difficult. My son had always stayed on top of his grades until this past year (freshman in high school). I had already decided not to take him on vacation but he was invited to go away with his cousins if I paid just the airfare and so I relented. What a mistake! He only had to miss three days of school and then the following week was vacation so all in all it was 12 days away for airfare, what a deal. No deal.
Long story short, became very ill and missed a full week of school a month before they were leaving, missed 5 days due to snow when school was closed (so no repercussions but the teachers had to double-time to make up for it, even though it is added on to the end of the schooyear), had an honors teachers that just didn't want to make this easy for him and in fact did grade him "O" on all homework that his missed but allowed him to make up one test I think, giving him an average of 63 for the semester -- good going teach! Sure taught him a lesson, don't ya think

.
So, no more time off for the kids once they are in 8th or 9th grade regardless of their abilities. Too chancy. They learn so much in so little time that it is too difficult to catch back up even if they are good students.
As for the younger kids, up to middle school or so, I personally think a family vacation is MUCH more a learning experience than the days in school they will miss. Of course, this depends on the child, but assuming no other obstacles, most kids in elementary school are learning not just basics, but social skills, as well, both of which can be taught outside the school walls.