Modern digital artefacts not only affect the way we work but increasingly also the way we learn and teach. The Internet, smart devices, as well as innovative forms of human-content interaction open up new pathways for successful knowledge transfer. The work presented in this paper aims to explore one of these recent developments, i.e. the use of Augmented Reality in school learning. We report on an experimental study in which two cohorts of secondary school students used a smartphone-based AR mobile-app to learn about astronomy and the solar system. The goal of the experiment was to evaluate the app's perceived usability and task load associated with a set of particular learning exercises. Results point to an average perceived usability and moderate task load, independent of the students' gender. Further, we found a negative correlation between the perceived usability and task load, even though this was only significant for male students.