The question here is twofold: what constitutes a "very good reason", and who makes that determination?
The United Kingdom has over forty thousand practicing Wiccans, Druids, and Pagans, so you've got to figure at least some of those folks have kids in school. If school officials start denying one group time out of classes for religious reasons, will all faiths be held to the same standards? When you have year-round classes, you've got to assume that at some point your term schedule is going to conflict with somebody's holiday observations. Will it be up to school administrators to decide whether one spiritual path is entitled to a day off, or that another is not entitled to the same privilege?
I'm curious -- how many of you have ever taken your children out of school for a Pagan holiday or other event? Did you approach it with school administrators beforehand, and how did things work out?