In large-scale educational assessments it is typical to analyse between-school differences (e.g. OECD, 2013) and look at how much of variance is explained by systematic school-level effects. In countries with relatively small differences between schools, a greater proportion of variance is located within schools, but systematic class or peer group -level effects are only seldom analysed due to sampling prodecures and lack of information regarding social relationships within classes. This is despite of the fact that it has been understood for decades that classmates and peers have a influence on each others' performance at school (Hanushek, Kain, Markman & Rivkin; Ryan, 2012; Song & Grabowski, 2006; Wentzell and Caldwell, 1997). In one of the very few studies that have reported on between-class differences in the context of large-scale assessment studies (Yang Hansen, Gustafsson & Rosén, 2014) it was found that compared to other Nordic countries, between-class differences are very large in Finland. However, cross-sectional studies cannot answer to the question whether these differences are a result of initial class composition or if the performance has developed differently, indicating that the difference may be partially produced by the classes (cf., Vainikainen, Hautamäki, Hotulainen & Kupiainen, 2015) . Therefore, longitudinal assessment studies are needed in order to understand the mechanisms of the development of performance.

The aim of the present study is to look at how class-level differences develop from the beginning of the fourth grade until the end of the sixth grade as measured by 10 to 12-years-old pupils’ performance in a large-scale assessment of domain-general learning to learn competences (see Vainikainen, Wüstenberg, Kupiainen, Hotulainen, & Hautamäki, 2015). More specifically, the research questions are:

1. How large are the between-class differences in Finnish schools at this stage?
2. Do the between-class differences increase when the pupils proceed towards the end of primary education?
3. Which factors explain the initial differences and the expected differentiating class-level development?    

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