In the last decade, there has been significant deployment of wireless in-class response systems that allow students to interactively answer instructors' questions. For example, more than 5000 instructors and 7 million students use such a system developed by iClicker Inc. Existing systems have three limitations which prevent them from achieving their full potential: (i) the cost of remotes, (ii) their need for batteries that often need changing, and (iii) misuse of the system by cheating students, who operate multiple remotes on behalf of absentees. We introduce an alternative, ID-Clicker, that addresses all three limitations. We modify commodity Radio-Frequency IDentification (RFID) tags to create battery-free and low-cost remotes. We further present an inference algorithm that analyzes students' responses and remotes' signal power similarity to detect students who cheat by responding on behalf of others. We evaluate ID-Clicker empirically and demonstrate its effectiveness in a classroom environment. We find that with a single RFID reader and four reader antennas, responses of remote pads generated from a whole classroom with a size of 7 m x 9 m area (covering 8 rows and 64 seats) are received with 100% response accuracy. Further, our system can detect cheating with high accuracy.