The College Board, which released the scores Wednesday, said the results reflect the record number of students from the high school class of 2011 who took the exam and the growing diversity of the test-taking pool — particularly Hispanics. As more students aim for college and take the exam, it tends to drag down average scores.

Still, while the three-point decline to 497 may look small in the context of an 800-point test, it was only the second time in the last two decades reading scores have fallen as much in a single year. And reading scores are now notably lower than scores as recently as 2005, when the average was 508.