Allegations of cheating in Philadelphia schools first surfaced in 2011, involving 53 district schools and three city charter schools.
In recent years, testing scandals have erupted in Washington; Cincinnati; Baltimore; Detroit; Houston; Los Angeles; and Newark, N.J., among other cities.
Since 2009, cheating has been confirmed in 37 states and Washington.
The Philadelphia scandal has drawn comparisons to one in Atlanta, where last year a Fulton County grand jury indicted 35 educators, including Beverly Hall, a former school superintendent. The allegations touched at least 44 schools.
Administrators, principals, and teachers were charged with racketeering, conspiracy, making false statements, and related offenses.
Six former Atlanta principals and teachers recently pleaded guilty to cheating offenses, bringing the number of pleas to 17. Prosecutors say Hall was a leader, driven by bonuses and federal funding tied to better test results.
The testing scandals have fueled a debate over using high-stakes classroom tests to judge the performance of teachers, administrators, and schools. In Atlanta, some teachers who cheated blamed extreme pressure to meet district goals.
Robert McGrogan, head of the union that represents Philadelphia school administrators, said he does not condone cheating under any circumstances. But, he said, during the years of the alleged cheating, the district was a pressure cooker.
"Do you know how many of us sat in meetings with our bosses and were told, 'You have to bring your scores up'?" said McGrogan, a principal at the time. "There was no how-to book given to us."
Supervisors warned principals that if they did not meet state standards, "you're not going to be a principal next year," McGrogan said.
On Friday, Mayor Nutter said that while the scandal reveals "immoral behavior," the district had made legitimate improvement in recent years.
"It does not take away the gains that have been made," he said.
On Thursday, the SRC fired Deidre Bennett, Michelle Burns, and Marla Travis-Curtis.
Bennett was principal of Cassidy Elementary, but had been a teacher leader at Huey Elementary. Burns, the principal of Kensington Urban Education Academy, had been principal at Tilden Middle. Travis-Curtis was principal of Lamberton Elementary.
Efforts to reach the three were unsuccessful.
City school officials said Friday that their own investigations into allegations of cheating were continuing.
In 2011, The Inquirer reported that dramatic test-score gains that began in 2009 appeared to have been achieved partly through cheating at Roosevelt Middle School in East Germantown.
The 53 schools that have been investigated for possible cheating - one in five of all district schools - come from every part of the city and span every grade level. Some had been labeled "Vanguards," a Philadelphia School District designation for its highest-achieving schools, and one that gave them flexibility in curriculum and budgeting.
A state-commissioned analysis of 2009 exams identified suspicious patterns of erasures at schools across Pennsylvania. Later, staffers and parents at Cayuga Elementary in Hunting Park told The Inquirer of cheating there.
The state Inspector General's Office later conducted investigations at 11 district schools - the "Tier I" schools, or those with the most serious allegations. The district investigated 19 Tier II schools and has yet to probe 22 Tier III schools.
Sixty-nine current and former employees were implicated in the highest-level, most serious investigations, School District attorney Jessica Diaz said, but discipline against them cannot proceed until the state releases its investigations to the district.
Of the 19 Tier II schools probed, three were cleared, no conclusion could be drawn at three, and cheating was found at 13.
Forty current district employees and 29 former employees were implicated. Twenty were administrators, 46 were teachers, and three worked in other capacities, including as counselors and a police officer.
Staff writer Craig R. McCoy contributed to this article.