Apparently don't make for slimmer students. I wonder if another tactic would work better: put them all on the school roof, on the far side from the roof entrance with no elevator access. Students wanting soda would have to climb stairs and get to the far side of the roof to get one. They may have to do so quickly to get done with lunch or back to class in time.
An added benefit would be the nurturing of entrepreneurial spirit: quick kids could buy extras and sell them at a profit to those too lazy/slow to make it.