I agree with most of what you say. But I also think homogenity is not the whole story. How many non spanish speaking kids are there in a typical Mexican classroom? I know Mexico does have an indigenous population, but I think the share of Mexicans who don't speak proper spanish is very low. Probably even lower than that of native swedish or russian speakers in Finland.

My conclusion is that economic and teaching related causes are probably more significant than ethnic ones. I think you would find that mixed language elite schools anywhere in the world rank higher than the average Finnish school. Sadly, it's a poignant feature of most european immigrant populations that they are poor and uneducated. They don't just speak different languages, they don't speak any language particularly well. It's just averaging of course but that's what the PISA test does.