Religious instruction rightly belongs with the families and not with the schools. As such, care must be taken that no particular religion be recognized above any other. Yet, it is reasonable for religious discussion to take place to understand the diversity of different cultures and families.
Staff and students with a bonfire need to be absent for religious reasons should be allowed to do so without penalty. However, that should be applied equally and reasonably to all religions, whether mainstream or not. This could be a difficult call, but it seems that most principals should be able to make that determination without prejudice when a family or staff member requests accommodation.
With such a policy in place, it would not be necessary to cancel school on Good Friday, as those who wish to attend mid-day services could simply request accommodation like any other religious holiday.