In the design of educational robots, it seems undecided whether robots should show social behaviors and look human-like or that such cues are indifferent to learning. We conducted an experiment with different designs of social robots, rehearsing the multiplication tables with primary school children in Hong Kong. Results show that affective bonding tendencies may occur but did not significantly contribute to the learning progress of these children, perhaps due to the short interaction period. Nonetheless, 5 minutes of robot tutoring improved their scores with about 30% and only for a few challenged children, performance dropped. We discuss that topics such as teaching language skills may be fostered by human likeness in appearance and behaviors but that for STEM-related subjects, the social aspects of robots hardly matter.