Schools teaching ordinary pupils are quite different. If you are worried about sending your kids to regular school, international or otherwise, you will have no problems.
Totally agree with you accept many state Polish English teachers that I know only have the basic English qualification, which is FCE (First Certificate of English) and this I find incredible that they are allowed to teach English to children when they only have the very basic knowledge themselves. Those of us who teach in private schools will recognise mistakes that state teachers make with children and question the pupil and get the response, “well my English teacher told me this”.
I once spent all weekend with my sister-in-law helping her with her English project, not doing it for her but teaching her; and when she went to present it to her English teacher, she accused her of cheating, because she had not taught her that grammar construction yet and was marked with a fail. What a load of crap!! You should always question the pupil and see if they understand what they are talking about and reward self-learning not put them off; but I found out the reason why. When it was my sister-in-law’s school open day I asked to come along, and we went from class to class and I kept my mouth shut; when we arrived at her English teaches class I introduced myself in English and you could have picked her jaw up from the ground she was so shocked. I went on to question her (smiling at the same time) and found her English was no better than my four-year-old daughter was at the time, and she had never been to the UK or any other English speaking country.
I am not saying that every state English teacher is the same, far from it, some of them are great, and I have had the privilege to meet many very good ones, but don’t assume things, always check out the teacher or school yourself. They will have nothing to hide if they are a good school!!