Note that she was not a university undergrad, but a grade-11 high school student, probably 16 or 17 years old. In the US, each year is a grade, and grade-12 is the final year of high school (which precedes university).

That only makes the work and paper more impressive, IMO. In my high school, our math teachers had very weak theoretical backgrounds. I suspect that Britney would have received less guidance in high school than would be available at university.