Unfortunately, it may be more common for girls to "dumb themselves down" in order to fit in. In my observation and experience, one helpful remedy is placement with intellectual peers, with whom they may confer, collaborate, and compete.

Until other parents respond, here are some links to resources and older posts on the forum, which may be of interest. They discuss various aspects of underachievement.
1) general description - http://giftedissues.davidsongifted....06/Re_WWYD_moving_abroad.html#Post232706
2) Hoagies's list - https://www.hoagiesgifted.org/underachievement.htm
3) Jim DeLisle article - https://www.davidsongifted.org/gift...erachievement-and-the-quest-for-dignity/
4) Jim DeLisle book - https://www.ascd.org/books/doing-poorly-on-purpose (via acsd, Association for Supervision and Curriculum Development, in support of the stated goals of public education)

In the event that a student may not be doing poorly on purpose, consider looking into Executive Function issues.

Parents may also want to consider the following: The Sept 2011 report summarizing research by NWEA - Fordham, titled "Do High Flyers Maintain Their Altitude?", indicates that some students may become "descenders" within a given one-third of their class. This information may have inspired some schools to intentionally create "descenders" as the school strives to close gaps and achieve equal outcomes.