Anna, I am just giving my perspective. Primarily, the curriculum is lacking. It follows the American curriculum yet the majority of the students are international- it needs to offer more courses to accommodate the population. But your American and you may be happy with it. My daughter's best friend goes to the American school and she tells her the maths they are learning are something just above a basic level. My eldest is learning maths of which I have absolutely no idea how to do.
My kids are half Polish so they go to a school that offers curriculum in Polish and English. It is not one of the ones listed above. We are extremely happy with it but we are also a bilingual family.
There is no perfect school. We have been thrilled with my children's school but I also know a French lady who was completely disappointed with it after having her children in the school for two years.
I also have an American friend who sends her children to the British school and will never send hers to the American school. They attended for one year until she found out that the American school has a charity status and pays no taxes as well as receiving monies from the U.S. gov't. However, the facilities are amazing and it is great to get to know other expats. I have many friends who have kids at the American school and they participate in lots of fun things but at the same time they are highly segregated from the rest of Poland. We don't want that.
Basically, it is not a simple decision, you need to find what works for you. The International American school is a joke- don't even look there. The so called Canadian school is run by Poles, one of them is married to an American- neither have ties to Canada but are using the name to make money.
There is a montesori school for older children here but I believe it is in the north end of the city.
Basically, for English speakers, there are only 2 schools in Warsaw : the British and the American Schools - all the others are expensive Polish private schools with poor English, poor management and underqualified, overworked, underpaid and therefore not staying long teachers and suitable only for some Polish snobs. I personally believe that only the British and the American Schools should be considered.
are you kidding me? no Polish snobs go the British or American schools? is this a joke?
the school my kids go to is managed well and the English is fine with a combination of native speakers and English speaking Poles- but it is not trying to be an English school it is a bilingual school.