The study examined measurement of internal efficiency in the school system: Focus on some selected Colleges of Education in Nigeria. This study adopted the ex-post facto research design. The population for this study comprised all the 70 public Colleges of Education in existence between 2012/2013 and 2015/2016 in Nigeria. There were 21 federal and 49 states owned Colleges of Education located in all the six geo-political zones of the country. The sample for this study comprises students of fiften Colleges of Education. The respondents will be all the 203 heads of departments drawn from these fifteen (15) Colleges of Education in three geo-political Zones. The three geo-political zones of Nigeria. The sample technique involved multi-stage, stratified random and purposive samplings. An inventory was used to collect data on number of students that progressed to higher level, number of stagnation, dropout and graduate outputs in each department in the Colleges of Education sampled within the selected sessions. Inventory titled “Measurement of Internal Efficiency of Colleges of Education inventory” (MIECOEI) was used to collect data from the respondents. The instrument was subjected to screening by experts in Educational Management and Test and Measurement in order to meet the face, construct and content validity. The inventory was administered through the help of the research assistants who distributed all the copies of the inventory to the heads of the departments in the Colleges of Education. In analyzing the data collected for the study, descriptive statistics such as percentages mean and cohort analysis were used to analyse the research questions. The findings of this research showed that there were increase in the rates of stagnation and dropout but increase in progression rate as students moved to higher classes and that the internal efficiency of the Colleges of Education was low. Based on the findings, it was concluded that there was low internal efficiency in the sampled Colleges of Education in Nigeria. It was therefore recommended that efforts should be made to maintain the increased in the progression rates within the first two academic sessions so that the increase in the rates of dropout and stagnation in the third year and among those that spent extra year (s) in the Colleges of Education would be reduced.