If you restrict the language of instructions to English or a foreign curriculum (IB may be arguably an exception), you naturally restrict the population of schools and so the population of students in the country. For top private schools where local students across the country compete for a slot, the selection process naturally leads to a strong student body and so a solid foundation to pursue rigourous education. That's my logic and why I much prefer noninternational private schools. Of course, the historical education path of the children matters. I can tell kids will be generally happier in Spain but I prefer bitter first sweet later.