We moved to Spain in the late 70's. It didn't even occur to us to send our kids to a spanish school.
It would have been just the same if we'd moved to France. I've worked, and we've lived in France, and its (ex) colonies, on and off, since the 60's.
So we put them in a very reputable international school in Spain, and have never regretted the decision, even though it cost quite a bit. Unfortunately the school couldn't provide education beyond age 16 for sciences, so they had to go back to UK after that.
Before leaving UK, we were told that our daughter would possibly get 2 GCSEs - she managed 9 "O" level GCEs at the international school, then went on to college in UK and an honours degree in Australia.
Our son also managed 8 "O" levels in Spain, 3 "A" levels in UK, and gained an honours degree in Engineering from UK, and a Masters in Wind Energy from Copenhagen.
If they had stayed in school in UK or had gone to a local school in Spain (or France) they would both probably be hanging around the street corners like so many of the youngsters these days.
OK, so are we rich b***ers who could afford to put our kids in a private school? No, not really, we just couldn't see a proper alternative.
On some great and glorious day the plain folks of the land will reach their heart's desire at last, and the White House will be adorned by a downright moron.
H. L. Mencken 1880 - 1956
Some may not like his views, but what a prediction!