The researcher aimed at investigating the effects of overcrowded classes on the teaching learning process at secondary level. By nature, this research study was descriptive so survey research design was employed and a closed ended questionnaire having twenty items related to the research problem were developed on five point Likert’s scale for the teachers (Senior Science Teachers) and the 10th grade students of government high schools from three tehsils of district Nankana Sahib. Pilot study was conducted to achieve the reasonable face and content validity and reliability, expert opinions and testretest techniques were used and 0.891 Pearson’s product moment was assured. One hundred (100) questionnaires for SSTs both males and female and 120 questionnaires for students both boys and girls were administered personally and with the help of some friends, six principals both of male and female were interviewed using structured interview schedule data collection technique and data was collected. On the basis of analysis and interpretation of data, indiscipline classes due to overcrowdings affect the teaching learning process badly; the performance of the students is not easily assessed in overcrowded classes and teachers are unable to interact with students easily in overcrowded classes were the conclusions of this research study. The researcher suggested that the government must make it possible to implement the law of recommended class size and reduce the problem by building more classrooms for the students at secondary level. Policy makers may formulate the policies for the effective assessment and evaluation process.