A former Florida prep-school administrator who posed as high-school students to take SAT and ACT tests as part of a nationwide college admissions scam pleaded guilty in Boston federal court Friday afternoon.

Mark Riddell, 36, pleaded guilty to conspiracy to commit mail fraud and conspiracy to commit money laundering in what investigators have called the “largest college admissions scam” ever prosecuted.

Prosecutors alleged over 50 parents paid admissions consultant William “Rick” Singer upwards of $25 million total to rig test scores, cheat on SAT exams, and bribe college coaches to secure their children spots at several elite universities. Many of the children were not aware of the scam.

While Singer is believed to be the engineer of the operation, prosecutors allege that Riddell was the brains—taking tests in students’ place or correcting answers after their tests were handed in.

“He was just a really smart guy,” Andrew Lelling, U.S. Attorney for the District of Massachusetts said at a March news conference announcing the charges from the FBI’s “Operation Varsity Blues.” “He didn’t have any side information about the correct answers. He was just smart enough to get a near-perfect score on the exam or to calibrate the score.”

Riddell, the former director of the IMG Academy, allegedly was paid $10,000 per test. That money was taken from “donations” parents made to Singer’s fake charity, Key Worldwide Foundation, which were also used to bribe Los Angeles and Houston test administrators to let Riddell pose as high-school students.

“I want to communicate to everyone that I am profoundly sorry for the damage I have done and grief I have caused those as a result of my needless actions. I understand how my actions contributed to a loss of trust in the college admissions process,” Riddell said in a statement last month, after the charges were announced.

According to the indictment, Riddell was not only able to ace the standardized tests, but he also made sure to get an appropriate exam score for each student so as not to raise the suspicion of test companies.

“Singer would discuss with parents what kind of score was impressive—but not too impressive,” Lelling said. “Then he would instruct Riddell to attempt to get that score, and he was just good enough to do it.”

Riddell helped the oldest daughter of Felicity Huffman, who allegedly paid $15,000 to have the 36-year-old correct her kid’s answers, giving her a 400-point bump. Huffman, 56, announced on Monday that she will plead guilty to bribery and fraud for her role in the scam.

“I am ashamed of the pain I have caused my daughter, my family, my friends, my colleagues and the educational community,” Huffman said in a Monday statement. “I want to apologize to them and, especially, I want to apologize to the students who work hard every day to get into college.”

Riddell, a Harvard graduate and failed tennis pro, has been working with authorities since February and agreed to the terms in a plea deal signed last month in exchange for his cooperation, “in the hope of obtaining leniency when he is sentenced,” the Massachusetts U.S. attorney’s office said.

“I will always regret the choices I made, but I also believe that the more than one thousand students I legitimately counseled, inspired, and helped reach their goals in my career will paint a more complete picture of the person I truly am,” Riddell said in the statement.

He now faces up to two decades in prison and a $250,000 fine, although prosecutors said Friday they are recommending 33 months for his July 18 sentencing hearing. His plea makes him the 18th person to plead guilty in the massive bribery scam.