Increasing affordability is making multi-tabletop spaces (e.g., in school classrooms) a real possibility, and first design guidelines for such environments are starting to appear. However, there are still very few studies into the usability of such multi-tabletop classrooms as a whole, considering well-established constructs such as cognitive load. In this poster we present an exploratory study of the usage of mobile eye-tracking techniques to follow cognitive load of a teacher during a lesson in such a multi-tabletop space. By analyzing several eye-tracking measures over three sessions of a collaborative learning lesson on fractions, we obtained insights on the user experience of the facilitator in these concrete sessions. We also show the potential of eye-tracking to identify critical episodes in the usage of a multi-tabletop space under realistic usage conditions, in a less intrusive and more objective manner.