The emergence of 3D printing has raised hopes and concerns about how it can be used effectively as an educational technology in school classrooms. This paper presents the results of a survey asking teachers from multiple grade levels and subject fields about the impact of 3D projects on student learning. Teachers were asked about the kinds of 3D projects they were doing with students and what skills or knowledge students were developing by participating in those projects. Participants reported that their students developed a number of skills while working on 3D printing projects, including 3D modeling, creativity, technology literacy, problem-solving, self-directed learning, critical thinking, and perseverance. Parallels between teacher-identified skills and widely cited lists of 21st-century skills suggest that 3D projects are a promising approach to preparing students for life and work in a digital age.