I definitely appreciate the importance of family. However, there are lots of ways to have "family bonding time" that don't involve a week long trip to Disney World when school is in session.
Many folks seem to be suggesting that, because I schedule our family vacations to coincide with schools' vacations that I am somehow putting family second. Frankly, I find that offensive.
For our family, going to a distant theme park when school is in session to save money/experience lower crowds/see the Halloween decorations just isn't an option. Especially amongst the DIS crowd, that's an unpopular position. But, it is a valid one that may not actually imply poor parenting skills on my part. I don't expect other families to make the same choices I make, nor do I suggest that it is the right/only possible decision for those families. However, if you choose to take your kids out of school, you have to accept the fact that there may be some consequences to doing so. At the grade school level, these might be exceedingly minor consequences. As the kids get older, things get more complicated.
In my opinion, the most important consequence is the long-term message I send to my kids about the importance of school. As a University professor, I'm big on school. Of course, it makes no difference to me if my own students come to lecture or not, so long as they can do the work. But, they have to be able to do the work, and the pedagogical research shows that actively participating in class is highly correlated with success.
In the original poster's case, it may not be a matter of convenience. It may truly be impossible to go to the World when schools are on vacation. If so, then s/he has a hard decision to make. If I were in his/her position, I'd exhaust every available option to go when school is out of session before considering going during school. Since we are able to go during "normal" vacation times, I don't know what I would do if there were no other option, but I suspect we'd put off Disney for another time.
As a foonote, please also recognize that asking teachers for assignments in advance is more work for them. True, teachers provide similar assistance during illness, but illness is unavoidable and unplanned. Not so a family vacation. So, when you do ask for the teacher's assistance, be prepared to be told no.