I suppose it depends on what the school class is doing during that time frame. What subject(s) is she missing? We homeschooled (not for gymnastics reasons) so I come from the point of view that learning can take place in many forms, environments, and at different times than the prescribed "school day". Of course, your school would need to be OK with this arrangement. Our school district would never have allowed this. Not even getting out just 30 minutes early.
As to why this is happening - I understand that an empty gym with less numbers in the training group provides a better training environment but frankly, I think the gyms/coaches also want to see which families are fully committed/invested. They willing to modify school schedules, have the ability to get the gymnast to the gym in the middle of the day (usually messing with their own work schedule), pay more for training (usually has more hours than the other groups), etc.