For some context, Fermat's last theorem has a beautiful statement, which can be explained to a high schooler, it appears naturally when teaching the pythagorean theorem and just a bit of generalization gets you all the way to its statement. Its proof uses connections between various parts of modern mathematics.
What about a theorem that goes even further, its statement is arithmetic, can be naturally motivated at a high school level and its proof goes into the realm of $\infty$-categories. Are there such statements? Maybe the answer is much simpler and the statement is Fermat's last theorem as I am not an expert.
For context, my experience with category theory comes from formal verification of software and modularity issues therein. While one might argue this can be motivated to a high schooler, the issues come in at a different scale. I would argue that these issues would show up in a well-designed computer science class for high schoolers, but that the objects in any concrete theorems directly depending on them will be much harder to motivate at this level without a course in rigorous formal logic or some amount of software engineering practice. Compared to these, in Fermat's last theorem all the objects and operations in the statement come without going beyond the standard curriculum.