Prosecutors said Singer's operation arranged for fake testers to take college admissions exams in place of his clients' children, and also bribed coaches to give admissions slots meant to be reserved for recruited athletes even if the applicants had no athletic ability.
Part of the scheme involved advising parents to pretend to test administrators that their child had learning disabilities that allowed them extended time to take the exam.
The parents were then advised to choose one of two test centres that Singer's company said they have control over: one in Houston, Texas, and the other in West Hollywood, California.
The test administrators in the those centres took bribes to allow Singer's clients to cheat, often by arranging to have a student's wrong answers corrected after completing the exam or having another person take the exam.
Parents paid tens of thousands of dollars for his services, which were masked as charitable contributions, prosecutors said.
Singer is scheduled to plead guilty on Tuesday in a Boston federal court to charges including racketeering, money laundering and obstruction of justice, according to court papers.
John Vandemoor, a former Stanford University sailing coach, is also scheduled to plead guilty to racketeering conspiracy charges.
Some 33 parents were charged, including Huffman, who is best known for her role on the television show Desperate Housewives, Loughlin, who appeared on Full House, and Loughlin's husband Mossimo Giannulli, a well-known fashion designer. Thirteen coaches and associates of Singer's business were also charged.
Huffman is accused of paying $US15,000 - disguised as a charitable donation - to the Key Worldwide Foundation so her oldest daughter could participate in the scam. A confidential informant told investigators that he told Huffman he could arrange for a third party to correct her daughter's answers on the SAT after she took it. She ended up scoring a 1420 - 400 points higher than she had gotten on a PSAT taken a year earlier, according to court documents.
Huffman also contemplated running a similar scam to help her younger daughter, but ultimately did not pursue it, the complaint alleges.
In many cases, the students were not aware that their parents had arranged for the cheating, prosecutors said.
On a call with a wealthy parent, prosecutors said, Singer summed up his business thusly: "What we do is help the wealthiest families in the US get their kids into school ... my families want a guarantee".
Reuters, Washington Post