Here in Quebec we have what is called Sport Etudes, this allows children who are at a certain skill level to do school in the morning and gym, or any sport, in the afternoon. The kids usually are around level 7 or P3 here. Only certain gyms/schools offer this system. It is certainly not just for Elites, but grades have to be kept up as the kids are tested academically.
At our high school, my DD next year could miss school to train, it is up to us, as long as her grades are kept up and she is training it is fine with them. We may take advantage of this one afternoon a week, only really to allow her to get home earlier than usual one day a week. Three days a week she leaves home at 7:45am and returns at 8:30pm, with homework on top of this it can be tiring for her and she is only in grade six.
Academically it won't be a challenge for her, but of course we would only do it if she kept up her average.
The French High School in town also allows another gymnast to skip phys-ed and English classes to train during the day.
Even Shawn Johnson attends regular school, is very bright and her school allows her to train when necessary and compete too. She maintains very high grades. She also only trains 25 hours a week, even leading up to the Olympics.
Quality time in the gym is so much more important than quantity.