1:15 PM PT -- William H. Macy just showed up at the federal courthouse in Downtown Los Angeles. His wife, Felicity, is scheduled to appear before a judge around 2 PM PT.

12:47 PM PT -- Singer has now pled guilty to charges including racketeering conspiracy and obstruction of justice.

9:15 AM PT -- The U.S. Attorneys Office broke down the alleged scheme during a news conference ... and named William Rick Singer as the ringleader. They say he created a fake non-profit, charitable foundation which served as a front to launder the payments from parents.

The feds said most parents paid between $200k and $400k for Singer's help. Some payments apparently went as high as $6.5 million. All told, they say investigators uncovered about $25 million in bribes.

Singer allegedly instructed the parents they could write off the alleged bribes as tax-deductible "donations" ... to his non-profit.

Feds say Singer will plead guilty to racketeering charges.

Felicity Huffman and Lori Loughlin have been charged along with 48 others in a massive bribery scam involving some of the most elite colleges in the country.

Court records show the actresses and others allegedly paid hefty bribes -- as much as $6 MILLION -- to get their kids into various schools, including Yale, Stanford, Georgetown and the University of Southern California.

The charges include conspiracy to commit mail fraud and honest services mail fraud. We've learned Felicity has been arrested and is presently in custody. We're told she will be released on a signature bond ... which essentially is a promise to appear in court.

The alleged scam was reportedly unearthed after authorities found a California businessman who ran an operation helping students get into the college of their choice. Authorities say parents would pay the man a predetermined amount which he would funnel to an SAT or ACT administrator or a college athletic coach.

The feds say if the money went to a college coach ... the coach would arrange a fake profile that listed the student as an athlete. If the money went to an exam administrator, the administrator would either hire a proctor to take the SAT/ACT or correct the student's answers.

As for Loughlin, she and her husband -- Mossimo Giannulli, the founder of Mossimo clothing -- allegedly paid $500,000 to have their 2 daughters designated as recruits for the crew team at USC ... despite the fact they did not actually participate in crew. However, the feds say Mossimo sent action photos of their daughters on rowing machines, including one of his older daughter on an ergometer to Singer via email.

Mossimo, who was also charged, took a cheap shot at one university. According to the indictment, he wrote an email saying he wanted to "make sure we have a roadmap for success as it relates to [our daughter] and getting her into a school other than ASU!"

Loughlin's daughter caught some flak last year when she posted a video about starting college. She stated, "I don't really care about school," and explained she was excited about partying and game days mostly -- which angered viewers who commented, calling her "spoiled" and "privileged." She later apologized for her "super ignorant and stupid" comments.

As for Huffman, she and her husband, William H. Macy, allegedly made a charitable contribution of $15,000 to participate in a college entrance exam cheating scheme on behalf of their eldest daughter.  The indictment says the daughter was given twice the amount of time to take the SAT as other students and the paid proctor agreed to secretly correct her answers afterwards.

The indictment says the girl received a score of 1420 on her SAT ... an improvement of approximately 400 points over her PSAT.

Macy was not indicted.

According to the docs, the FBI recorded phone calls in which Loughlin and Huffman talked about the scheme with a witness who was cooperating with authorities.

And, the federal indictment out of Boston has a twist ... it says most of the kids admitted to the schools had no idea their parents allegedly greased the wheels to get them in.

We reached out to the reps for both Felicity and Lori ... so far no word back.