I'm giving a talk soon to a group of high-school students about open problems in mathematics that high-school students could understand. To inspire them, I would like to give them examples of high-school students who have made original contributions in mathematics. One example I have is the 11th-grader from Hawai'i named Kang Ying Liu who in 2010 "discover[ed] nine new geometric formulas for describing triangle inequalities."
Do you have any other examples of high-school students who have made original contributions in mathematics?
I'm not sure this is really what you're looking for, but Britney Gallivan, then $16$, disproved the famous claim that it was impossible to fold a piece of paper in half ten times, by folding one twelve times. She also came up with a model that correctly explained the limit, and predicted how big the original paper would have to be to be folded $n$ times.
Archive of page about Gallivan from the Pomona Historical Society