During the past few decades, the measures of reducing the class size are prioritized in the educational policies of different countries as one of the practices that should improve the quality of educational outcomes (Bascia & Fredua-Kwarteng, 2008). Hence, a number of studies have examined the effects of class size on educational outcomes, in order to determine how reducing the class size contributes to minimizing the negative consequences of social inequality, improving student achievement, enhancing the basic skills and competencies of students in the first few years of schooling, etc. (Achilles et al., 2002; Biddle & Berliner, 2002). While Graue and Rauscher (2009) define the class size as the actual number of students in a class assigned to an individual teacher, in some studies student-teacher ratio is used as the measure of class size. Student-teacher ratio is calculated by dividing the total number of students attending a school with the total number of school’s employees, including teachers who work full- and part-time, the principal, and school specialists, and excluding the teachers who are absent for longer than one school term (Blatchford & Mortimore, 1994; McRobbie et al., 1998; Wilson, 2002). Based on the analysis of a pilot-program of class size reduction and of the existing literature on class size and student-teacher ratio Sharp (2002) estimates that in US primary schools class size on average exceeds student-teacher ratio by 9 to 10 students. A number of studies conducted in USA (e.g. Chargois, Irons & Carlson, 2011; Hanushek, 1994; Chingos, 2010; Ehrenberg et al., 2001; Becker, 2006), as well as those in which European countries are included (e.g. Wössmann & West, 2006), have tried to empirically test the relation between class size, student-teacher ratio and student achievement, yielding contradictory and inconsistent results. In Croatian primary schools, there were no comprehensive studies of the relation between these variables.

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