The development of classification operations among middle school students was examined. A random section of 510 boys & 510 girls in five socio-economic regions of Tehran were selected by multi stage sampling and two piagetian classification operation tests were administered to them. Findings showed: 1) A low achievement rate of the final stage of classification operation and the cumulative frequency in the transitional stage; participants did not achieve the base success rate of 75%. 2) There were no significant gender differences in these aspects of the tests. 3) Mean scores of students in the under privileged regions (9, 13, 16) were lower than those of students other regions (3, 6). 4) A comparison between Iranian students and their international counterparts revealed a three-year delay among Iranian students. This delay can be explained by the inadequacies and shortcomings of Iran's formal education and school system in encouraging spontaneity among children.