You can see the detailed question results at http://www.scribd.com/doc/58246319/Brown
The summary is:
82% of test-takers correctly answered the "easy" multiple-choice question (identify the case from a quote) about Brown. 51% correctly answered the "medium" multiple-choice question (identify the case that Brown overturned).
The "hard" question was an essay question that asked "describe the conditions that this 1954 decision was designed to correct." The results broke out as:
47% Inappropriate 26% Partial 2% Complete 23% Omitted 3% Off task
That 2% Complete is the number the article quotes. Which I would describe as "2% of students had enough time, writing ability, and understanding of Brown v. Board of Education to complete a short-answer essay question under time constraints and to the full satisfaction of the scorer."