This is a new favorite story to share with students: a high school student recently published an elementary proof of Cayley’s Formula from graph theory.
Professional article in the American Mathematical Monthly (requires a subscription): https://www.tandfonline.com/doi/full/10.1080/00029890.2025.2590423
By coincidence, this is one of the math theorems that was cited in the movie Good Will Hunting.
Mean Green Math 