Post's Matthews ranks area schools; PG's best is 56th, 2nd is 102nd; Many in separate "Catching Up" group; WHY so low?

Story: Finding the better high school

Individual school rankings: Challenge Index 2010; The Local List
Prince George's best high school is 56th of 172; 4 PG schools are in the bottom 10%; 9 were excluded from the rating and listed separately among failing schools that are "catching up."

School district rankings: Challenge Index2010; The Local List; Top Districts in D.C. Metro Area
Prince George's comes in 21st of 28

Maryland has recently bragged about having some of the best schools in the nation.

Election after election, candidates keep telling us that they are "for quality education," or "for better schools," etc. 

So why are Prince George's schools among the worst in the area and and among the worst in the state, year after year after year?