We are on the Thanksgiving cruise this year, 11/23/02, and my son will miss 2 days of school. Granted he will be just in 1st grade, but we also try to minimize days missed, especially consecutive days. He attends a private school where expectations are very high and I don't feel comfortable with him missing whole weeks of school for a vacation. Granted it is tempting to save on the money, but we took advantage of off-season travel when he was little, and we will again when he goes to college. Education is #1 in our household, and we choose to cough-up the extra $$ to go in the summer or over holidays.
We normally do WDW the week of New Year and he misses one or two days also, so that's already 4 days missed for vacations.
Good luck with your decision.