BOSTON (AP) — In the wake of a massive college bribery scheme, the schools caught in the middle have been left facing a thorny question: What to do about the students who may have been admitted through fraud?

The University of Southern California announced late Monday it had placed holds on an undisclosed number of students, meaning they can't register for classes or obtain transcripts until their cases are reviewed. After a review, USC officials said they would take appropriate action, "up to revoking admission or expulsion."

At Yale, the president declined to comment on specific cases but said it's a "longstanding policy is to rescind the admission of students who falsified their Yale College applications." Stanford similarly noted that students could be "disenrolled" or have offers of admission rescinded.

More than 30 parents have been charged in the scheme in which prosecutors say a disgraced college admissions consultant, William "Rick" Singer, took millions of dollars in bribes to assure their children's entry into top colleges by getting them recruited for sports they didn't play and by arranging for standardized tests to be rigged.

Federal prosecutors say some students never knew about the bribes and fraudulent entrance exams that got them into some of the nation's top universities, but investigators say some students did and were even involved in submitting false information about athletic feats in their applications.

One student posed for photos in 2017 that were allegedly doctored to make it look like he played water polo, according to court records. Investigators say the family bought a water polo ball and cap on to be used for the photos. The photo was edited and used in a fraudulent "athlete profile" that helped him get into USC. His father, Devin Sloane, is accused of paying $200,000 for the scheme. Messages left for Sloane were not immediately returned.

"The parents, the other defendants, are clearly the prime movers of this fraud. It remains to be seen whether we charge any students," U.S. Attorney Andrew Lelling in Boston said in announcing the charges last week.

Several schools said they are still considering what to do about students whose admission may now be tainted.

At Georgetown University, the indictment cast a shadow over the applications of about a dozen students. The school said it was examining its records and "will be taking appropriate action." An alumnus started a petition Tuesday urging the school to pull admissions from anyone involved, saying their presence on campus would diminish the school's prestige and encourage cheating.

Wake Forest University said a student mentioned in the indictment remains enrolled, and school officials have no reason to believe the student was aware of the alleged crimes.

Only USC has said definitively that it revoked offers of admission from applicants tied to the scheme. More than half the parents charged were trying to bribe their children's way into the school in downtown Los Angeles.

Elizabeth Heaton, a college consultant and former University of Pennsylvania admissions officer, said any student who knowingly applied through fraud should be expelled. But schools face a dilemma with students who may not have known, she said, and it may be challenging to prove how much a student knew.

"It's possible that some of these students were as duped as the colleges were," said Heaton, vice president of Bright Horizons College Coach, a consultant firm near Boston. "Do you just summarily punish the sons for the sins of the fathers? It's hard."

The scheme was remarkable in part because of its scale and the cast of rich and famous defendants, including actress Lori Loughlin and her fashion designer husband, Mossimo Giannulli, who allegedly paid $500,000 to have their two daughters labeled as crew team recruits.

But colleges are hardly unfamiliar with efforts to bend the rules. An entire industry has been built to catch people who cheat on the SAT and other standardized tests.

The same day that prosecutors in Boston revealed details about the bribery ring, in fact, their counterparts in Los Angeles announced arrests in a scheme to help Chinese nationals obtain student visas. Prosecutors say five people used fake passports to impersonate the Chinese nationals and take an English fluency test on their behalf.

Mark Sklarow, CEO of the Independent Educational Consultants Association, said schools are increasingly on the lookout for applicants coming from China with faked transcripts, recommendations and other application records.

"It's an issue that I think every college in America has been dealing with over the last few years," Sklarow said.

In cases from abroad, colleges typically don't have to deal out justice on their own: The people involved are often deported or denied visas. But in other cases, colleges have been swift to boot students who get in through fraud.

Cornell University expelled a student in 2013 after finding that she transferred with a forged transcript showing a 4.0 grade point average, when in fact she had a 2.79. The student, Cavya Chandra, was sentenced to five years of probation for loan fraud in that case because she had received federal student aid.

More recently, colleges have drawn attention for using their power to rescind admission in cases unrelated to fraud.

Harvard University ignited a debate in 2017 when it rescinded offers to at least 10 students because they made offensive comments online. The school found that students had joked about the Holocaust, sexual assault and racial minorities in a private Facebook group. Some observers called it a violation of free speech, while some applauded Harvard for taking a strong stance against racist comments.

In 2017, the University of California at Irvine revoked admission to nearly 500 applicants because more students had accepted than officials expected.