Well, the teachers usually have a masters degree and it is quite a popular profession, so it is somewhat hard to get into the University to be a teacher.

Finland is also very egalitarian, or socialist if you will. This means that there are not so many good or bad schools, all schools have pretty much the same resources. Of course there are differences, but nothing as flagrant as in the US.

Private schools are practically non-existent (less than 2%).

So while the Finnish system lifts the average, I'm not sure how much the top performers benefit from it.