This study investigated the impact of teacherís subject matter knowledge and their behavior on the studentís performance. Qualified and well behaved teachers help to boost learnerís confidence and involvement in learning activities. In Pakistan, the quality of school level education is continuously deteriorating. It may be because of the reason that most of the private schools, due to their financial constraints, hire less qualified and untrained teachers. The data was collected through a 5 points Likert scale questionnaire. Responses were obtained from the students of secondary and matric classes of three different schools. Data was analyzed using various statistical techniques, (factor analysis, regression analysis, mean comparison). The analysis of data revealed a significant relation between the student performance, teacher subject matter knowledge and teacherís behavior. Besides this, it was found that the achievement level of students of high income area is better than that of low income area. Furthermore, the subject matter knowledge and behavior of the teachers of high income area schools were found to be better than those of low income area. This study suggests that the subject matter knowledge and behavior of the teachers should be given prime consideration while hiring them. In-service teachersí training programs must also be held to keep teachers updated in the subject. Personality development and motivational trainings should also be held to help teachers in improving their behavior. Besides this the perks and privileges should also be raised for the teachers to attract knowledgeable and well behaved people to the teaching profession.