If you take PISA scores to be a good indicator than there are homogeneous and non-homogeneous societies near the top and bottom.
Canada ranks right up there with Finland every year http://en.wikipedia.org/wiki/PISA sometimes topping Finland in a category. It is easy to find countries that are culturally homogeneous near the bottom of the rankings as well.
Does being officially bilingual, northern, and woodsy have an impact on PISA rankings?
Finns do not make up a large or even a moderate proportion of Canada's population.
If I had to guess: yes. I honestly do believe that winters and environmental controls (like pavement) affect your approach to a wide variety of problems, and are immersive enough to do so on a nationwide scale. Not so sure about language, but a lot of other people seem to be.
This might manifest in educational systems more prominently than other parts of culture, or it might not. The thread of discussion is mostly about why the US is feeling inferior to Finland in one particular aspect, and grandparent stated that Canada also tests better there, so I would figure there might be something that Canada and Finland have but the US (and UK?) don't. Like, say, some aspect of culture.
Although obviously bordering cultures tend to affect each other greatly, my first instinct is always to look at a nation's climate.
One of the top Canadian provinces educationally is British Columbia. The majority of the province lives in Vancouver and Victoria. Those two cities have an extremely mild climate that is similar to Seattle and bears no resemblance to Finland or the rest of Canada.
If climate was the underlying reason for education going well in Canada, then you would not expect British Columbia to do as well as it does.
(Disclaimer. I'm biased because I grew up in Victoria.)
Destroying this hypothesis entirely, though, we have the high educational achievement of Singapore, which is hot and humid as heck every single day of the year.