Here's an overview of some of the most shocking instances of teacher cheating, plus a few episodes that may have been overblown.
The 'Lake Wobegon' Effect (1987-89)
One of the earliest investigations of teacher cheating was spearheaded by John Jacob Cannell, a family physician from West Virginia who was shocked to hear that his poverty-stricken home state, with high rates of illiteracy, was performing above the national average  on standardized tests. Cannell latched on to the issue and discovered that students in 48 states were supposedly performing above the national average—in part because they were being judged using out-of-date comparisons.
This phenomenon was christened the "Lake Wobegon Effect," after Garrison Keillor's legendary town where "every child is above average." Cannell's reports argued that score inflation resulted from infrequent test updates and too much "teaching to the test," as well as outright teacher cheating . While his findings were hotly debated, a Department of Education-sponsored study confirmed most of them .
Columbus, Ohio: After President Clinton Lauds School, Students Claim Cheating (2000)
Just weeks after President Clinton visited a Columbus school to laud its astronomical gains on test scores  and argue that Clinton-Gore strategies were working , the school was enveloped in a cheating scandal . Three students told a teacher that they had received assistance on the previous year's exam . District investigators found no evidence to support the claims, but some parents found the accusation credible, and the veteran teacher who passed along the students' claims said she had been forced to go on disability leave after retaliations from the principal .
New York City: Early Cheating Scandal May Have Been Overblown (1999-2001)
Aggressive schools investigator Edward Stancik  uncovered a wide range of cheating in New York City schools, including a seventh-grade teacher who had placed the answers to a test by a pencil sharpener, encouraged his students to sharpen their pencils, and left the room. But Stancik's most explosive findings, which implicated 32 schools and 52 educators , did not hold up to scrutiny. A New York Times investigation into his methods found that some of his accusations seemed dubious and that innocent teachers may have suffered as a result .
Chicago: 'Freakonomics' Author Catches Cheating Teachers (2002)
"Freakonomics"  author Steven Levitt and fellow economist Brian Jacob developed a method to screen tests for teacher cheating  by looking for identical strings of answers. Their conclusions, based on Chicago public-school tests from 1993 to 2000: Cheating on standardized tests occurred in at least 4 percent to 5 percent of classrooms every year ; teachers in low-performing classrooms were likelier to cheat; and a "pronounced spike" in cheating occurred when Chicago introduced high-stakes testing in 1996. As a result of their report, Arne Duncan, then CEO of Chicago Public Schools and now U.S. Secretary of Education, asked the economists to put their algorithm to work catching cheaters in action . The experiment worked: When students of teachers suspected of cheating took the tests again, their scores dropped.
Birmingham, Ala.: School Targeted Students to Withdraw Before Tests (2004)
When the director of a GED program in Birmingham noticed that many students were showing up at his office weeks after they had "withdrawn" from a local high school because of "lack of interest," he decided to investigate. With the help of a school board member, he found that more than 500 students—about 5 percent of the student body —had been asked to leave the school. These forced withdrawals happened before students were to take an important standardized test but after the school was evaluated for the funding it would receive based on enrollment. The school district denied that the withdrawals had anything to do with getting rid of students who might have dragged down the school's test scores.
Texas: 700 Schools Flagged for Potential Cheating; State 'Investigated' With Survey (2004-07)
When the Dallas Morning News analyzed test results across Texas, it found hundreds of schools with test scores that had jumped and dropped in suspicious ways . The newspaper identified low-income schools with students at one grade level who struggled with basic skills—and students in the next grade who received nearly perfect scores or outperformed the state's most elite districts.
One elementary school's students scored so well that Oprah Winfrey featured it on a special about schools that "defy the odds."  But a teacher said her high-scoring students could barely write their own names , and when the same students went on to middle school, their scores plummeted. The state ultimately decided to investigate at least 700 schools. But for more than 600, the "investigation" consisted of simply asking schools to fill out a survey about their testing procedures . Taking many schools at their word, the state declared that the vast majority were innocent despite further evidence that some schools cleared of wrongdoing had actually cheated .
Los Angeles: Charter Founder Orchestrated Cheating at Six Schools (2011)
The director of a group of six charter schools in Los Angeles ordered his principals to break the seals on state tests and help students prepare for the exams with actual test questions . When teachers reported  the order, the school's governing board demoted the director but did not fire him, noting he had "expressed very, very deep regret." When the Los Angeles Board of Education voted to shut down the charter schools completely , parents, students and teachers made passionate arguments  for keeping the schools open under different leadership  and eventually succeeded .
Atlanta: Teachers Changed Answers in a District 'Run Like the Mob' (2011)
Teachers in Atlanta were so used to changing students' answers on standardized tests  that they gathered for "erasure" parties and prepared answer keys on plastic transparencies to make the cheating easier. One teacher told investigators that she feared retaliation  if she didn't participate, saying the district was "run like the mob." At least 178 teachers and principals have been implicated in the scandal, which was first brought to light by the Atlanta Journal-Constitution .
Washington, D.C.: Investigation Ongoing at 'Blue Ribbon' School With Suspicious Erasures (2011)
There's still no conclusive evidence of cheating at a Washington, D.C., school that gained federal accolades —and monetary bonuses—for its high performance on tests. But a USA Today investigation found that student test sheets had unusually high numbers of wrong answers that had been erased and replaced with right ones. Testing experts said the odds that these erasures occurred purely by chance were smaller than the odds of winning the Powerball grand prize in the lottery . (District officials say teachers trained students in testing techniques that may have led to more erasures.) The school's former "poster boy" principal recently resigned  from his position as a superintendent. Michelle Rhee, the former chancellor of D.C. schools who touted the school's success, has resisted answering questions .