General pedagogical knowledge (GPK) is a central component of teacher knowledge. Teacher education programs in many countries therefore provide corresponding opportunities to learn (OTL), and in-school experience is regarded as a core component of OTL fostering knowledge in the area of general pedagogy. However, empirical research on the effectiveness of school experiences during teacher education does not tell us precisely how different kinds of OTL are related to GPK. This paper first reports on the conceptualizing of the GPK test in the context of TEDS-M. Then the relationship between practical in-school OTL of German and US future primary teachers and their GPK is investigated. On the basis of results from Latent-Class Analysis using two core indicators of in-school OTL (the length of time spent on teaching students and the extent of being supported by a mentor or supervisor), three types of future primary teachers in both the US and Germany are distinguished: “starting” (type 1), “autonomous” (type 2), and “balanced” (type 3). In both countries, type 3 future primary teachers reported that they had had OTL to reflect on and improve their teaching to a larger extent than the type 1 teachers. Type 3 teachers also generally achieved better GPK test results than type 1 teachers. Furthermore, there is also a tendency that type 3 future teachers show better results than type 2. Based on these results, we hypothesize that the quality of future teachers’ activities during in-school practicum matters with regard to important outcomes of teacher education, making in-school OTL an effective component of teacher education. Research findings are discussed with regard to the relationship between theory and practice during teacher education.