Student participation in class is an important aspect of the learning experience, and can provide valuable feedback for teachers. However, getting students to interact in large classrooms is challenging. This paper presents SpeakUp, a mobile temporary social media app, confined in time and space that aims to improve interactions in classrooms. In SpeakUp, teachers can create temporary chat rooms accessible to students located nearby. Students can then anonymously post messages, which can be rated up or down by others. Our evaluation results (with a class of 300 students over the course of a semester) show that temporary social media can be used as an effective tool to improve classroom interactions by providing: (i) confined classroom-like here & now interactions to foster adoption, (ii) anonymity to increase participation and (iii) social media rating for highlighting interesting content. Students appear to use SpeakUp not solely for asking questions but also as a general backchannel, which can result in spam.