I know a bit about Portugal, not much about Greece and Slovakia. To summarize, my kids had way better state K-12 education in Palo Alto, CA and Westfield, NJ than their Portuguese cousins had in state schools in comparably affluent places in Portugal. When we moved to Philly, our daughter could not apply to one of the magnet schools there because she wasn't a resident yet, so she attended a private school. Those state magnet schools are better than any state school in Portugal, and indeed better than most private schools in the US. The ordinary Philadelphia city schools are terrible, but so are the Portuguese schools for areas as relatively deprived as non-Center City Philadelphia.