"We do not include any magnet or charter high school that draws such a high concentration of top students that its average SAT or ACT score exceeds the highest average for any normal-enrollment school in the country. This year, that meant such schools had to have an average SAT score below 2000 or an average ACT score below 29.3 to be included on the list."
So in other words this 'list' actually selects for schools with dumber students and excludes top performing students/schools.
"The Challenge Index is designed to identify schools that have done the best job in persuading average students to take college-level courses and tests. It does not work with schools that have no, or almost no, average students. We put those schools on our Public Elites list."
Maybe they should name the list 'schools that force the greatest number of students to take college tests' - thats all this is.
source
http://www.washingtonpost.com/local/education/abcs-of-americas-most-challenging-high-schools/2013/04/11/ca4f27aa-a2fb-11e2-82bc-511538ae90a4_story.html