In this essay Tatiana Geron argues that classroom "crowdedness"--the spatial, temporal, and group dynamics of many students interacting in a shared space--shapes teachers' every day ethical decision-making and should be essential to an ethical theory of teaching. Drawing from Philip K. Jackson's ethnographic work and her own teaching experience, Geron identifies features of the classroom that contribute to its ethical complexity--size, compulsory closeness, diversity, temporal pressures, and group dynamics--and lead to three types of challenges for teachers: linear challenges, or ethical difficulties that scale up with classroom size; second-order challenges that arise from the unintended consequences of teacher decisions; and integral challenges that arise from the group dynamic as teachers and students interact as ethical agents. Geron uses a case study of teacher decision-making to explore these features and the potential they create for novel resolutions to ethical dilemmas in the classroom.