Fostering pro-environmental behavior to achieve a sustainable society is one goal of Education for Sustainable Development worldwide. Connectedness with nature positively correlates with pro-environmental behavior and therefore needs to be studied in detail. In this mixed-method study, applying the “Inclusion of Nature in Self” (INS)-scale, we investigated 1) how closely preadolescents from urban middle schools (n = 651, 6th grade) are connected with nature, 2) whether the type of school (general or academic track) or 3) the time spent outdoors influences students’ connectedness with nature. We also explored 4) students’ reasons for their specific INS level and 5) how reasons and levels interconnect. Data show that students’ reported nature connectedness differs significantly with school type and that the reasons for feeling connected to nature are diverse. Positive attitudes and emotions toward nature plus time spent outdoors seem to predict high connectedness with nature, indicating the importance of direct nature experiences.