What is the highest high school diploma you can get?

There isn't a hierarchical ranking of high school diplomas in the way there might be with college degrees. A standard high school diploma is the most common and widely accepted. Some schools might offer diplomas with distinctions (like "high honors" or "honors") based on GPA, but these are still all considered high school diplomas, not different *types* of diplomas. There's no single diploma that's objectively "higher" than another.