It is if you opt into the school system ghosty.
I don't think there are any claims that half a day will make a huge difference to his education. My concern would be that the message - to the younger daughter as well as the teenage son - is that education, school, turning up, call it what you will, is optional, is not really important, does not really matter.
People are being really dismissive of schools here (only half a day, much of what they do doesn't matter) and yes in the context of education as a twelve year career that's true. If education was withdrawn - or as it used to be was only the premise of the wealthy - I wonder if people would still state that.
I would not consider myself someone who sees education in itself as 'vital'; I have no qualms about letting my child have a day off if unwell or tired or in one case, sad - but for a hair appointment, even I am 
It isn't about him missing vital parts of his education at all but the overall outlook. And I don't much care for the way those of us who disagree have been painted as uptight and anal miseries - with awful hair, natch. Mines pretty nice actually!
Lastly in at least one case on here - and I am well aware it is a specific problem of dyslexia - the writing style is not fluid or accurate. In the context of a chat forum that doesn't matter a bit. In the context of applying for jobs, university and similar, it does. Half a day will make no difference to that of course but let's stop with the pretence that education is unimportant. If the parent cannot educate - and in at least one case on these boards it would be reasonable to assume she can't - then the teacher has to. But the child needs to be there in order for that to be done!