Quotes from HP Lexicon:
QUOTE
Q: Can American kids go to Hogwarts ? (Kelly)
A: No, they have their own school. You'll find out in Book 4. Hogwarts just serves Britain and Ireland.
QUOTE(2)
A school in Brazil from which Bill Weasley had a penfriend(GF7)
QUOTE(3)
Durmstrang Institute
Headmaster: Igor Karkaroff
Location: a castle in northern Europe, far enough north that days are very short in winter.
QUOTE(4)
Where is Beauxbatons?
During the feast celebrating the arrival of the students for the Triwizard Tournament, Rowling chose to serve "bouillabaisse". This is a very typical fish soup from the southeast of France and is not really eaten elsewhere, at least not as a common dish. It is likely, then (though an assumption) that Beauxbatons is on the Mediterranean coast of France (maybe by Marseilles) which could explain as well why the students of Beauxbatons are so cold in Hogwarts. -- thanks to Amyll.
So there are Magic schools all over the globe but imagine how hard it would be for JKR to try ad learn the slang and accents for every country if students from all over the world went to Hogwarts. It was much better having only the three schools, any more would have made the books too cluttered with useless dribble.
Personally I would have liked to see an Australian in the books but since the story is set in England/Scotland/Ireland/Wales there was no need to add any other places in. I would assume that since France and Northern Europe (?) are close to England, JKR chose to have schools from there to compete in the Tri Wizard Cup. And those accents are easy to mimic.