It's sad how stupid reporters report wrong "news", the error gets repeated all over the Internet, and finally lands in Slashdot whose editor didn't know the original news report was wrong.
The 16-year-old girl did not invent a previously-unknown theorem. What she did is to re-invent a theorem which Euclid already listed and proved over two thousand years ago (http://aleph0.clarku.edu/~djoyce/elements/bookIII/propIII9.html). But Euclid listed hundreds of theorems, most have simple and basic proofs, and most of them are never specifically taught. In this case, the girl was not taught this theorem, but she thought that she could have used such a theorem in her homework, so she went about proving it (with help from her teacher, who was also not familiar with Euclid's mention of this theorem).
The girl's proof is different Euclid's, but still very simple and elementary, and is in no way a profound addition to Mathematics. But this girl is still admirable, in that she had the creativity and resourcefulness to imagine a "new" (to her) theorem, and to go around proving here - rather than sticking to the "cheat sheet" of theorems she was taught in class. This girl definitely deserves an A in her math class, but not worldwide mention on news classes.
Of course, it's not her fault, but rather that of the reporters who blew this story out of proportions, and reported this stuff as a new theorem, a breakthrough, or other irrelevant adjectives - without checking the validity of this "story" with any Mathematician worth his salt. This "story" should never have made headlines, and definitely not slashdot. But the girl still deserves praise, and of course an A :-)