It's not like she is the only one that gets it. Diva get's it too. It's part of the hibernation circle it just varies in how long it takes for them to remember. And through that time they have to be "sheltered" so to speak.
Diva had it easy being pampered while Saya, as Red Shield's weapon, had to be watched because of that slight berserk stunt she had in Vietnam. Or how else do you explain that Julia is the fucking doctor Saya always goes to get her blood transfusion?
Her going to school is just keeping the image up because it would look very suspicious if a girl her age wouldn't go to school.