[Math] Research done by high-school students

Question

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?

Answer 1

In 1988 in the IMO, Australia decided to use the following question:

Let $a$ and $b$ be positive integers such that $ab+1$ divides $a^2+b^2$. Prove $\frac{a^2+b^2}{ab+1}$ is a perfect square.

The problem was proposed by Stephan Beck, West Germany. No one in the committee was able to solve it. Two of it's members were George Szekeres (Erdős number of 1) and his wife, both famous problem solvers and problem creators. The problem was then sent to 4 prominent number theory researchers and they were asked to work on it for six hours. None of them could solve it in this time. The problem committee submitted it to the jury of 19th IMO marked with a double asterisk, which meant a super-hard problem, possibly too hard to pose. After a long discussion, the jury finally had the courage to choose it as the last problem of the competition.

Eleven students gave perfect solutions. The solution to the question used a new technique in problem solving that had never been used before. However 11 high school students were able to surpass prominent number theorists in their own field by solving the question. The technique used for solving the problem is called Vieta Jumping.