Researchjournali’s Journal of Education 2015, Vol. 3(7)This study sought to determine if there was any difference in academic achievement between two categories of students enrolled at different times in Tanzania by employing both quantitative and qualitative methods. Quantitative analysis indicated that the second category performed significantly lower in their examinations than the first category; That is, -18.8>-1.96 for a two tailed Z-test. The same category of students performed lower than the first one when the examination results were segregated according to the year of study or subject-wise. Qualitative data indicated that deviant behaviors undesirable for students learning were associated with greater number of students from the second category than the first one. Thus, the researchers highlighted that enrolling all students at once should be viewed as one of the important strategies for improving the performance of the secondary school students.