Classroom assessment techniques (CATs) are ungraded activities in a classroom setting that provide feedback to the teacher and to the students themselves, on the current state of student learning and understanding, which can subsequently drive corrective actions where necessary. Student response systems (SRSs) provide a technological solution for CATs whereby students can respond anonymously and instructors can provide instant feedback. However, existing systems have tended to suffer from constrained input, limiting the quality of the student responses. In particular, existing SRSs typically only employ well known form based input metaphors such as the multiple-choice selection and text-box input. These input types are not well suited to responses that require significant graphic or symbolic elements such as equations, circuit diagrams, and other drawings. These SRSs also have logistical issues in relation to portability and ownership of the equipment. In this paper, the authors present an SRS designed from the ground up to support CATs with freeform input to fulfil the needs of the science, technology, engineering and mathematics (STEM) classroom, though the solution is applicable to any learning environment in which freeform input is valuable. To mitigate logistical issues, the solution employs touch based Android tablets and smart phones commonly owned by students and a freely downloadable student app. This paper details the design of teacher and student interaction, including instructor preparation prior to class. The authors also examine some of the issues surrounding freeform graphic and symbolic input on a range of device form factors and the particular solutions that they found effective. A summary of their ongoing evaluation of this system is also outlined within.