I'm pro-immigration, in the classic "the more the merrier" tradition of those with confidence in both markets and human innovation.
But to be intellectually honest, we have to acknowledge that immigrants of different languages, cultures, and economic backgrounds will bring many 'average' statistics of education (and health, etc.) down. This is even true if the immigration is a net-benefit to both natives and newcomers.
With that in mind, the massive downvoting of the parent comment (at -9 at one point) is a dishonest enforcement of political correctness.
Even if you disagree with the conclusions of the linked article, and the other views of that article's author, the comment here is factual and relevant to the topic at hand, and the linked article includes other useful facts about Finland.
If the submission question "What makes Finnish kids so smart?" is legitimate at all -- the study of why one country has different results than others -- discussion has to be free to consider major things that make that country different from others. In Finland's case, that includes its traditional and apparently legally-enforced homogeneity.
I would not want to live in such a place, but I don't want my understanding of such places crippled by a long list of "things you can't say".