A dynamic student-teacher interaction during class is an important part of the learning experience. However, in regular class settings and especially in large classrooms, it is a challenging task to encourage students to participate as they tend to be intimidated by the size of the audience. In this paper, in order to overcome this issue, we present SpeakUp, a novel context-aware mobile application supporting the social interactions between speakers and audiences through anonymous messaging and a peer rating mechanism. Context-awarness is achieved by bounding interactions in space and time using location-based authorization and message boards with limited lifetime. Anonymity is used as an icebreaker, so students dare writing down any question that pops in their heads. Peer rating is used to make it easy for teachers to access the most relevant ones and address them. We performed an evaluation with 140 students over five four-hour lessons that indicate that SpeakUp is easy to use and is perceived as useful.