Paper interfaces merge the advantages of the digital and physical world. They can be created using normal paper augmented by a camera+projector system. They are particularly promising for applications in education, because paper is already fully integrated in the classroom, and computers can augment them with a dynamic display. However, people mostly use paper as a document, and rarely for its characteristics as a physical body. In this article, we show how the tangible nature of paper can be used to extract information about the learning activity. We present an augmented reality activity for pupils in primary schools to explore the classification of quadrilaterals based on sheets, cards, and cardboard shapes. We present a preliminary study and an in-situ, controlled study, making use of this activity. From the detected positions of the various interface elements, we show how to extract indicators about problem solving, hesitation, difficulty levels of the exercises, and the division of labor among the groups of pupils. Finally, we discuss how such indicators can be used, and how other interfaces can be designed to extract different indicators.