You can pick your weights more carefully. Did you try it yourself? Maybe 10 minutes isn't enough for the current population of students to master the hand-eye coordination required. I'd record their results and give them another 10 minutes and make it an experiment about how fast they mastered physical tasks involving their bodies. Maybe calculate the standard deviation of all groups results after 10 and 20 minutes.This is an experiment that they used to inflict on KS4 kids, when I was teaching. I never quite understood how it was supposed to be run and teachers used to bully the kids into believing that it was meaningful and that they could actually get results. We had a tray of kit, as described and the kids would have a happy ten minutes, whirling the masses around, hitting each other in the face by mistake and then writing up the experiment according to the teacher's instructions. The problem is that the situation is not stable. The restoring force is Mω2r and, with any value of Weight is either too high or too low to obtain a steady value of r. The experimenter has to beat this using skill, luck and imagination. Otoh, using a Force Meter instead of the selection of hanging masses can produce a measurable relationship between restoring force, r and ω. Using a motor with variable speed gives a chance of reasonable results. In fact, a potentially good investigation.