The principal of an Atlanta school where cheating was rampant pleaded guilty Friday, saying both his boss and Superintendent Beverly Hall put extreme pressure on him to falsify standardized scores.

Christopher Waller, the former principal of Parks Middle School, will become an important witness for prosecutors in their case against Hall and other former educators who have maintained their innocence.

There are now 13 defendants left in the case who haven’t pleaded guilty and would face trial in May. Twenty-one defendants have pleaded guilty.

“I orchestrated cheating by several teachers and fostered a culture of cheating,” Waller said as he teared up during an apology. “No matter how much we actually improved the school, there was no way that we could keep up with targets. The targets were unattainable, at least not by legitimate means.”

Waller will testify during a trial that his area superintendent, Michael Pitts, told him to keep his mouth shut about cheating, or else Waller could lose his job, said Deputy District Attorney Fani Willis. Waller also will testify that Hall rewarded him with praise and first-class plane flights on trips, despite her knowledge about complaints of cheating at the school.

He pleaded guilty to a felony charge of making false statements, and he agreed to cooperate when the case goes to trial.

Waller avoided prison time as a first-time offender. Instead, he must pay $50,000 in fines and restitution, serve five years of probation, perform 1,000 hours of community service and apologize.

Friday was the last day a judge will accept negotiated plea deals. Any defendants in the widespread Atlanta Public Schools cheating case who don’t make guilty pleas by the end of Friday will likely go on trial this spring, including Hall and senior members of her staff.

Sandra Ward, a former administrator at Parks Middle School, also pleaded guilty to a reduced charge of misdemeanor obstruction. She was sentenced to serve one year probation, repay $5,000 she received in bonus money, perform 250 hours of community service and cooperate with the prosecution.

“The leadership at APS was focused more on improving test scores than helping the students learn,” Ward said while reading her apology in court.

A 2011 state investigation said “erasure parties” took place under Waller’s supervision at Parks Middle School, where teachers would gather to correct students’ answers on standardized tests so the school could meet its academic goals. The school’s students scored unbelievable results on their standardized tests since 2006, shortly after Waller took leadership of the school.

Waller told investigators at the time he wouldn’t have cheated because he didn’t want to risk losing his $107,000 annual salary and because he was a minister.

The former educators who pleaded guilty have been required to serve probation, do community service, repay bonus money and cooperate with prosecutors.

Those who go to trial will face up sentences of up to 20 years if found guilty on racketeering charges.