As a math teacher I strongly take the opposite side. I have seen students come back from missing only one day and are lost. They cannot do the new work because doing it depends upon what they learned the previous day. After a week of vacation it is almost impossible to get back on track.
Family vacations are great but they have a time and a place. If parents cannot take off when school is not in session parents must find other ways to bond with their children. There are always weekends and evening. Even before all the testing, the stakes were just too high and no one will convince me that vacations should take place on school days.