Public schools are getting tougher on absences because of the No Child Left Behind Act. Schools that have attendance that falls below 95% stand to lose funding.
In our school district, they have enacted a policy that any number of days missed that totals more than 15 will require a doctor's note. They are also less likely to excuse absences that are due to family vacations.
A child cannot make up work that they miss during an unexcused absence. That means that tests, quizes, homework and projects that are given during that time period can be recorded as a "zero" in the grade book. This is, of course, at the teacher's discretion. Most are not that hard-nosed about the rule. The rule has been enacted to discourage the practice, not punish the child.
Frannn, if I were you, I would make it a point to discuss this with your DD's teachers and principal. Most are very understanding about vacations during the school year. They aren't happy about it but they know that sometimes there are no choices for a family.
Make every effort to get any assignments before you leave and be prepared to tutor your DD in whatever areas that she may have trouble with when she returns. Call her teachers when you get back and make sure that you have her missed assignments. Middle schoolers are very - shall we say - spacey? My own DD, an excellent student up until 6th grade, fell apart that year (ah, those hormones!). It has taken until 8th grade to pull it back together! Your DD will need you to keep up on what she has missed and what needs to be done.
Ultimately, the decision is up to you. If she's already going to miss 8 days, one more is not going to make that much of a difference. Since your question is not whether you should take her out for those 8, but whether one more would hurt her, I say just go for it and be prepared to face the consequences when you return.