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Studying English in Krakow, private tutors, schools or universities [49]
just make sure they're NATIVE speakers of the language
Yes, and above all teachers, not just somebody who can chat in their own language.
Are they worth considering or they're just usual schools?
Their website looks good and they seem to be respectable (and part of the relevant international bodies), They're a school though, as in school to send your kids to full time, rather than adult learners studying part time.
This is probably by far the best bet in Krakow: britishcouncil.pl/en/english/schools/krakow
They have very high standards about the teachers they recruit (and are part of the British government). Very often their staff are also textbook writers and teacher trainers. Usually the British Council is the best option (though a long way from being the cheapest).