All our 'holidays' as children were taken in the holidays, mostly it was day trips. My second daughter once had 2 days off (Fri. & Mon.) because her father was running in the Paris half-marathon and she was approaching French GCSE (there is a segment on Paris). She accompanied him, learned a bit about Paris and improved her French. That is, I think, acceptable and the Head agreed.
As a teacher, the complexities involved with children taking time off are, I assure you, huge. The curriculum is broken down into syllabuses; which are broken-down into smaller units; which are broken down into even smaller units; which are broken-down into (you've guessed it!) individual lesson content.
An extreme example (which is one I have personal knowledge of) was a Pakistani girl (12 years old) who had been in the UK for almost 2 years, but whose reading age in English was 7 and a bit - we had instigated intensive tuition since she arrived some 8 mths. previously and got her going. (Her family were fairly recent arrivals, spoke no English and she could manage fine in the community she lived in.)
She did not arrive for my intensive 1-to-1 sessions in September and eventually we discovered that she was back in Pakistan. Her parents had taken her back 'for a holiday' before the end of year and she finally appeared towards the end of October. I immediately tested her reading age and she did not even register on the scale at 5 years old.
This is the sort of thing that makes it a farce. Children must be in school. Sorry if you have to have camping holidays as we had to do. Lessons don't get taught twice!