Could she be going underground? Here's a link to an old Linda Silverman article about very young girls dumbing down to fit in as early as kindergarten.
http://www.jeffcogifted.org/images/jagc/pdfs/where_have_all_the_girls_gone.pdf Does the other kid who is having issues have similar abilities? If so, maybe you could ask the teacher to give the two of them little challenges together. If she had a buddy who was doing the same stuff and getting a little bit of special attention, she might show her true colors.
