We evaluate the impact of joint-liabilityincentives in the classroomusing a randomized field experiment.The instructor designsgroups of three studentsin the classroom and providesa premiumto their homework's gradeonly if all three members of the groupmeetsome requirements. To isolate the joint-liability effect from selfish motivations, we also design an individual incentives treatment. We find that joint-liabilityincentives impact positively on the grades attained in homework and midterm exams both in experimental coursesand in other courses taken by the students in the semester.Though the average positive effect seems to disappear in final exams, the overall impact of joint-liabilityincentives on the academic achievements in the semester is still positive. A drawback of this program is a decrease in classmate satisfaction. The significant effectiveness of the peer monitoring developed by joint-liability incentivesin a group providesnovelimplications for the design of grading policies in the classroom and for other social settings where incentives may be based in peer monitoring or joint liability.