An online learning evolution and systems of its management exploitation have led to the possibility of accumulating data on the education process. The accumulated data provide a basis for analytics and subsequent decision-making of online teachers and university administration. Researchers utilize educational data mining for different purposes by means of searching templates in these data. A field connected with controlling quality of fulfillment of free response assignments has remained largely untouched, though. Decision support for online teachers, implementing this procedure, allows to reduce the influence of the human factor, associated with over-loading and time constraints, and enhance the control quality. To carry out such support for online teachers, an approach and intelligent decision support system, developed with the usage of mining data being stored in a learning management system when controlling quality of fulfillment of free response assignments, are proposed in the paper. Based on experiment results, it was discovered that the system detects errors in performed free response assignments missed by online teachers, which proves its efficiency and usefulness for education process.