The research project presented in this article was designed to provide a better understanding of the stable and significant differences in the PISA results between two otherwise very similar Nordic welfare states, Denmark and Finland. In the PISA studies, Finnish students repeatedly achieve the highest Nordic (and partly worldwide) scores in e.g. reading, science and math, while Danish students score lower. Even though Denmark has one of the world's most expensive educational systems, the OECD ranks the Finnish school system as the world's best both in terms of quality and equity (OECD 2004). "The basic research question is why these differences continue to persist". The case study methodology was mainly inspired by Kirsti Klette's classroom research (Klette 2003) which involves both interviews and observations. Thus, the overall design could be labeled "mixed methods" (Johnson and Onwuegbuzie "Educational Researcher", 33(7): 14-26, 2004). Five regular public schools in each country were sampled for the comparative classroom studies. The basic theoretical approaches follow Hundeide's theory of pedagogical communication and relations (2003) and Csikszentmihalyi's flow-theory (1992). Both this study and statistical studies (Sorensen 2008) show that the difference in the Danish and Finnish PISA results mainly consists in the relatively better score by the lowest scoring 25% of the Finnish pupils compared to the lowest scoring Danish quartile. The results of this study point to a number of possible classroom related reasons why the Finnish school system can produce a better outcome for the lowest scoring quartile of pupils. These reasons are presented and discussed in the article. The study underlines the need to focus more on good classroom management in Denmark--and recommends further international, comparative research in order better to understand the huge differences shown in large scale international programmes such as PISA, PIRLS and TIMMS. The study also reveal the need for more knowledge about inclusive classroom practices, the use of teacher assistants and free, healthy school meals for all pupils.