Last Thursday, D.C. Mayor Vincent Gray and officials from D.C. public schools and the Office of the State Superintendent of Education (OSSE) held a news conference to crow about results on tests administered in April. Proficiency rates on both reading and math sections of the D.C. Comprehensive Assessment System exams were up slightly for many grades in schools across the district, the mayor said.

He took credit for those improvements – a 2.8 percentage point jump from 2011 to 2012 in the number of children proficient in math, a 0.5 point increase in reading proficiency. The results “are proof positive,” he said, that a program he introduced to make pre-kindergarten more widely available is already having an impact.

Over the past five years, proficiency gains are even larger – up 18.1 percentage points in math and 9.5 percentage points in reading since 2007. But when you look closely at the school-by-school results also released last week, some are discouraging, particularly at the very schools celebrated in 2008, 2009 and 2010 for their soaring proficiency rates. Several schools have seen proficiency skid alarmingly, dropping by half or more while the rest of the district was inching upward.

That’s evidence that adults – principals, test coordinators, perhaps teachers – cheated at some schools in those years when proficiency rates climbed. Then, when Chancellor Kaya Henderson and OSSE introduced tighter security measures in 2011, it became more difficult for the adults to cheat, and the kids’ scores and the schools’ proficiency rates dropped from their prize-winning peaks.

Consider, for example, Noyes Education Campus in Northeast Washington, whose former principal Wayne Ryan was richly rewarded for boosting the school’s scores in his eight years as principal. He and his faculty twice were given generous bonuses for their scores — $20,000 total for Ryan, $16,000 for each teacher.

The data released last week suggest again that Noyes’s scores were manipulated. In 2008, 61.5 percent of students tested at Noyes were proficient in reading, 57.7 percent proficient in math. The 2009 scores were even higher: 84.2 percent proficient in reading, 62.8 percent in math. Then, in 2010, proficiency rates began to drop, and on the most recent tests fell way below the district average: only 31.6 percent of Noyes test-takers were proficient in reading, 33.2 percent proficient in math.

So what happened? My theory is that it became harder for adults to take possession of the answer sheets, erase the students’ wrong answers and change them to right. CTB/McGraw-Hill, the company that developed and scored D.C. tests, flagged Noyes for having statistically irregular numbers of wrong-to-right erasures in 2008, 2009 and 2010, a sign of cheating. (Disclosure: My wife, Linda Mathews, led USA TODAY’s investigation of erasures, published in March 2011.)

Last week’s data show the same phenomenon at two other schools once celebrated for their high scores. Aiton Elementary’s proficiency rates peaked at 58.4 percent in reading and 57.9 percent in math in 2008, then tumbled. This year, reading proficiency stood at 17.5 percent, math at 20.6 percent. At J.O. Wilson, where every classroom was flagged for erasures in 2010, only 46.5 percent of test-takers were proficient in reading this year, and 39.5 percent were proficient in math. That’s down from 72.3 percent in reading and 76.6 percent in math three years ago.

Pardon all the statistics, but there is no other way to solve the puzzle D.C. officials ignore at their news conferences. When I looked at two other elementary schools, Powell and Smothers, with similarly large numbers of low-income students but very few wrong-to-right erasures, I did not see big declines.

Gray and school officials are aware of these numbers, but the statistics don’t fit their optimistic story line. The D.C. inspector general and the U.S. Department of Education, charged with investigating erasures on past D.C. tests, should consider not only why scores went up but also why they have dropped at once-celebrated schools. That’s the real story, the one that should be shared with D.C.’s parents, teachers and students.