It's being described as the biggest college admissions fraud scam ever prosecuted by US federal authorities.

Key points: Test administrators allegedly took bribes to allow clients to cheat, including correcting wrong test answers

Test administrators allegedly took bribes to allow clients to cheat, including correcting wrong test answers In some cases, the faces of children were Photoshopped onto images of athletes to exaggerate athletic credentials

In some cases, the faces of children were Photoshopped onto images of athletes to exaggerate athletic credentials Prosecutors have named 33 parents and 13 coaches so far, but more could be charged

At least 50 people have been arrested over a $35 million scheme to help wealthy Americans cheat their children's way into elite universities such as Yale, Standford, Georgetown, UCLA and USC.

Nine athletic coaches and 33 parents, most of whom are prominent in law, finance and fashion industries, were charged.

Dozens, including Desperate Housewives star Felicity Huffman and Full House actress Lori Loughlin, were arrested on Tuesday.

"These parents are a catalogue of wealth and privilege," US attorney Andrew Lelling said in announcing the fraud and conspiracy investigation dubbed "Operation Varsity Blues".

"For every student admitted through fraud, an honest, genuinely talented student was rejected."

Sorry, this video has expired Andrew Lelling says there can be no separate system for the wealthy (Photo: AP).

How does the alleged scheme work?

Enter: William "Rick" Singer.

Federal prosecutors in Boston charged Mr Singer for running the scheme through his Edge College & Career Network.

It relied on bribes, phoney test takers and even doctored photos depicting non-athletic applicants as elite competitors to land college slots for the offspring of rich parents, prosecutors said.

Part of the scheme involved advising parents to lie to test administrators that their child had learning disabilities that allowed them extra exam time.

Parents were advised to choose one of two test centres that Mr Singer's company said it had control over: one in Houston, Texas and the other in West Hollywood, California.

The company charged up to $2.5 million per child for the services, which were masked as contributions to a scam charity.

The test administrators in those centres allegedly took bribes of tens of thousands of dollars to allow Mr Singer's clients to cheat, often by arranging to have wrong answers corrected or having another person take the exam.

Mr Singer would agree with parents beforehand roughly what score they wanted the child to get.

Prosecutors said the scheme began in 2011 and helped children get into elite universities. ( AP: Damian Dovarganes )

Mr Singer also allegedly helped parents stage photographs of their children playing sports or even Photoshopped children's faces onto images of athletes downloaded from the internet to exaggerate their athletic credentials.

On a call with a wealthy parent, prosecutors said, Mr Singer summed up his business.

"What we do is help the wealthiest families in the US get their kids into school … my families want a guarantee," he allegedly said.

In many cases, the students didn't know their parents had arranged for the cheating, prosecutors said, although in other cases they knowingly took part.

None of the children have been charged.

What are Loughlin and Huffman alleged to have done?

Ms Huffman's husband William H Macy (left) has not been charged. ( Reuters: Mike Blake )

Court documents said Ms Huffman paid $15,000 that she disguised as a charitable donation so her daughter could take part in the college entrance exam cheating scam.

Ms Huffman, a former best actress Oscar nominee, is married to fellow actor William H Macy.

Court papers said a cooperating witness met with Ms Huffman and Mr Macy at their LA home and explained to them that he "controlled" a testing centre and could have somebody secretly change her daughter's answers.

Lori Loughlin allegedly gave $500,000 to have her two daughters labelled as recruits to the USC crew team. ( Reuters: Mario Anzuoni )

The person told investigators the couple agreed to the plan. Mr Macy was not charged, but authorities did not say why.

A Los Angeles judge ordered the Desperate Housewives star be released on a $250,000 bond.

Ms Loughlin, best known for her role in the sitcom Full House and the recent Netflix sequel Fuller House, is married to clothing company founder Mossimo Giannulli, who was also charged in the scheme.

Mr Giannulli was also charged in the scheme but was released on a $1 million bond.

Prosecutors alleged the couple gave $500,000 to have their two daughters labelled as recruits to the rowing team at USC, even though neither participated in the sport.

Their 19-year-old daughter Olivia Jade Giannulli, who has a popular YouTube channel, attends USC.

Ms Huffman, Ms Loughlin and Mr Giannulli are yet to enter pleas.

Which universities were implicated?

The coaches worked at such schools as Yale, Stanford, Georgetown, Wake Forest, University of Texas, USC and UCLA. A former Yale soccer coach pleaded guilty and helped build the case against others.

Who's been charged?

Prosecutors have named 33 parents, 13 coaches, and associates of Mr Singer's business, but said the investigation continued and more parents and coaches could be charged.

Other than Mr Singer, Stanford's former sailing coach John Vandemoor and the college-admissions consultant at the very centre of the scheme pleaded guilty to racketeering conspiracy charges on Tuesday in Boston.

Three other cooperating witnesses, including a former head coach of women's soccer at Yale, have also agreed to plead guilty, prosecutors said.

Others appeared in court and were released on bail.

What punishments are they facing?

The alleged masterminds of the scam and parents who paid into it could all face up to 20 years in prison if convicted.

Prosecutors said it was up to the universities to decide what to do with students admitted through cheating.

Yale University and USC said in separate statements they were cooperating with investigators.

Reuters/AP