Massive Open Online Courses (MOOCs) have received much attention in higher education; however, evidence about MOOCs at the K-12 level is scarce. To shed light on the phenomenon, we use the i-MOOC that aims at fostering upper secondary level students’ information literacy. The i-MOOC is a blended MOOC developed and refined in a design research process; it meets established criteria for high-quality MOOCs. In 2020, 1032 upper secondary level students in German-speaking Switzerland took the i-MOOC; the sample comprises N = 167 students who voluntarily filled in a questionnaire. The students are mainly from high schools and vocational schools. Learning effects are captured with a performance test. Information literacy gains are significant and medium in size: d = 0.75. The technology acceptance of students is evaluated using the extended unified theory of acceptance and use of technology (UTAUT2). Student technology acceptance of K-12 MOOCs is primarily driven by hedonic motivation, i.e., perceived fun and entertainment. However, this type of motivation negatively predicts learning gains. Implications for teachers and educational decision makers are discussed. •A systematic literature review on empirical evidence for K-12 MOOCs.•A conceptual framework for the quantitative evaluation of K-12 MOOCs.•Empirical evidence for a K-12 MOOC at upper secondary level in Switzerland.•Based on our evidence, the validity of student evaluation of K-12 MOOCs is doubtful.•Based on our evidence, hedonic motivation is a negative predictor for learning gain.