The brain is a funny, funny thing, and all sorts of bizarre cognitive and memory dysfunctions and adaptations have been documented. While I'm skeptical about this particular story due to the source, the description of the events does seem plausible to me.
Even "amnesia" is not just the simple loss of memory as portrayed in movies, but a complex phenomena with several sub-categories.
http://en.wikipedia.org/wiki/Amnesia
Note also that while she may be
remembering stuff from childhood, it's quite possible that these memories are not completely accurate, or even real. One thing you should recognize about memory is it's incredible plasticity and adaptability. The brain doesn't record events, like a video camera, so much as build models of them; networks of sensory images, emotions, sequences of events, and associations with other similar memories. And these models can be influenced and reshaped by further experience, even to the point where you
remember things that never happened (eyewitness testimony is generally considered among the weakest forms of legal evidence for this reason, for example).
So I would suspect then that what she is really experiencing is an assembly of specific real events and bits and pieces from other sources, such as generalized recollection of her daily routine at that age and general knowledge of how schools operate taken from experience, TV, reading and other sources.
For some unknown reason her brain suddenly decided that the last 17 years never happened, and that she's still a schoolgirl, and as a consequence re-adapted itself to provide her with an environment of memories, images, and feelings that fit that paradigm. How much of this consists of things she
actually experienced at that age is unknown.
