It's a fluff piece.

The real story here is that a 10th grader, after using a Theorem that wasn't taught in class, was encouraged to prove it - which she did, successfully. The teacher then sent it to a few academics who were thought that was a rather impressive accomplishment for a 10th grader, so they wrote her some encouraging words. That's it.

The theorem and its proof are in Euclid's Elements, (Book 3 Proposition 9: http://aleph0.clarku.edu/~djoyce/elements/bookIII/propIII9.h...)