This paper analyses the influences of parental expectation on middle school students' cognitive development, a subject that is seldom touched in Educational Data Mining (EDM) studies. Despite its anonymity in EDM literature, parental expectation plays a significant role in education. Even though parental expectation is widely studied in the domains of education and psychology, present models are too simple to specify its working path. As the influence of parental expectations is difficult to quantify, we conducted our investigation in one middle school in the city of Hangzhou by applying a set of questionnaires and cognitive assessments. Through the investigation, we verify an inverted-U-shaped relationship between parental expectation and cognitive performance under the moderation of family function. This result extends the EDM study by analyzing the family's contribution and provides a superior model of the impact of parental expectation.