Automatic Assessment Systems empowered by mathematical engines allow the development of online assignments for Mathematics, which goes beyond multiple-choice modality. Automatically assessed assignments, used with formative purposes, can support teaching and learning from several perspectives, such as conceptual and procedural understanding, metacognition, enactment of adaptive strategies, and teachers' management of the class. This paper reports on an experimentation where automatic assessment has been used in a blended modality according to a model of formative assessment and interactive feedback to enhance learning. The experiment involved a total number of 546 students of 8th grade in the town of Turin (Italy). The use of the automatic assessment is shown and exemplified. Data from learning tests, questionnaire and platform usage are analyzed and used to show the effectiveness of the interactive materials for enhancing mathematical understanding and self-assessment skills. Moreover, a profile of the students who did not use the online opportunities, defined as "reluctant users", is drawn and discussed.