Academic achievement has a crucial role in students' education, especially during their high school years. In many countries and specifically in Turkey, students' performance in the exam for higher education is the biggest determinant for their admission to a college. There might be many factors that can affect students' achievement and class size is a very important one which is the focus of this study. The purpose of this study was to find out if there is a significant correlation between number of students per classroom among 81 cities in Turkey and each city's student achievement ranking in the Transition to Higher Education Examination which is abbreviated as "YGS" in Turkish. In the study, the data of the number of students per classroom among 81 cities in Turkey and the data of each city's achievement rankings on Transition to Higher Education Examination (YGS) were used. The Spearman's Rho analysis was conducted in order to examine the correlation between class size and achievement ranking. The result of the Spearman's Rho correlation was -.366 with .001 significance level. This significant negative correlation between class size and achievement ranking indicated that the cities which have a low achievement ranking on the YGS tend to have greater number of students per classroom. Result of the study suggests promoting classes with lower number of students in a classroom so that students would not be affected negatively in terms of their academic achievement. (C) 2015 The Authors. Published by Elsevier Ltd.