What makes a high school challenging according to The Washington Post?

I've been reading about The Washington Post's lists of 'America's most challenging high schools', but I'm still kind of unsure what exactly is their criteria. What factors do they consider when compiling these rankings?

The Washington Post's annual "America's Most Challenging High Schools" list rates schools primarily through an index formula that's a simple ratio: the number of Advanced Placement (AP), International Baccalaureate (IB), and Advanced International Certificate of Education (AICE) exams given at a school each year, divided by the number of seniors who graduated that year. A school's ratio is then used to determine its rank on the list.

That said, to be included in this evaluation at all, the school has to have at least a certain percentage of students whose families qualify for lunch subsidies. This is designed to avoid the possibility of just having well-resourced schools with relatively rich students dominating the list. Also, to ascertain equity, they examine the percentage of students who receive scores on these exams that are high enough to likely earn them college credit.

It's important to note that while this methodology does reflect a school's commitment to pushing students to a higher level of coursework, it doesn't fully encompass every factor that contributes to a school's overall quality or rigor. Factors like class sizes, the breadth and depth of course offerings, student-faculty ratio, or support for students with different learning needs won't necessarily show up in this type of ranking.