That's like asking if someone can afford it SHOULD they go on a long flight to an expensive hotel for a holiday.
There are lots of environmental reasons that no doubt you could raise about people taking long unecessary flights. There are also ethical issues around human rights, political corruption and exploitation of labour in many countries that are tourist destinations.
In the end though, everything we do has some potential 'externality' effect on other people and we would end up living in a cave and eating gruel if we follow that arguement to its ultimate end.
So long as I pay my taxes and pay for my childrens' schooling myself - I think the externality effect is relatively small and it comes down to personal choice. If I had great state grammar schools in my area that is where my DSs would be going and I woudl save the money.
Funny how a large proportion of the teachers at my DSs private school also send their children there on a 50% staff discount. Not all teachers it seems are against private schooling.