Fox entered the plea in Boston's federal court Friday.
The 62-year-old had previously agreed to plead guilty to a racketeering conspiracy charge in a deal with prosecutors.
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Authorities say Fox brokered bribes to help wealthy parents rig their children's college entrance exams.
He was also accused of arranging bribes to get two students admitted to the University of San Diego as recruited athletes, and one to the University of Texas.
He has agreed to forfeit $245,000 received through the scheme.
Fox's attorney, David Gerger, sent ABC13 the following statement:
"Martin is a good person who has helped a lot of students over his lifetime. But what he did here was wrong; he takes full responsibility for it; and he is going to do everything he can to make it right."
Fox's charge carries a sentence of as many as 20 years in prison, but prosecutors are recommending a punishment at the low end of guidelines.
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The Associated Press contributed to this report.