Effectiveness of teachers has a crucial place in education and it is usually evaluated based on students' achievement. The focus of teacher effectiveness has been primarily on quality of teachers and how teachers instruct. However, there are external factors that influence teachers' effectiveness like the number of students per teacher. The goal of this study was to identify if there is a significant correlation between number of students per teacher and students' achievement. In the study, the data for the number of students per teacher was obtained by dividing the total number of students in high schools by the total number of teachers in high schools in every city of Turkey. The data for students' achievement was based on each city's achievement ranking on Turkey's Transition to Higher Education Exam. Spearman Rho's analysis was conducted to see if there is a correlation between these variables. The result of the analysis showed a significant correlation of -.561. This moderate negative correlation between the student teacher ratio and achievement revealed that the cities with greater number of students per teacher tend to have a low achievement on Turkey's Transition to Higher Education Examon. The result suggested more teachers should be hired in order to decrease the number of students per teacher so that students' achievement can enhance. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.