The paper proposes a machine learning model that can automatically identify off-task behaviors of students while using an intelligent tutoring system. Only log files that record students' actions with the system are used for the development of the model. The model utilizes a set of time features, performance features and mouse movement features and is compared to i) a model that only utilizes time features, ii) a model that uses time and performance features. In order to address data sparseness problem, a robust Ridge Regression algorithm is designed to estimate model parameters. An extensive set of experiment results demonstrate the power of using multiple types of evidence as well as the robust Ridge Regression algorithm.