What Maximilian means by grounded is that Lady can only go to school and straight back home every single day for one whole month.
That means no afterschool get-togethers, no detours, no playing outside, and no shopping at the bakery to grab some cookies. It sucks. Not only is she getting rid of the chance to develop her social life, but she is also being deprived of leisure time. To make things worse, she has to study right after getting back, and she has to do it until dinner arrives. It really, really sucks. There is no way she will be able to keep up with all of that.
And more than anything, such a strict schedule might hinder Lady's scheme.
