Some colleges also avoid vetting applicants online because it can be difficult to authenticate that an account belongs to the student in question. “This is a land mine for admissions officers,” said Bradley S. Shear, a lawyer who specializes in social media and privacy. He added that treating one group of students differently than another group could be potentially discriminatory. “That’s why admissions officers have to be very careful if they decide to look up an applicant online or review their social media profiles.”

Yet students and their parents have no way of knowing whether admissions officers at certain colleges look at applicants’ online footprints — or when a college rejects a student based on something found online — because colleges and universities generally don’t post information about their social media vetting practices on admissions sites.

“When they take a college tour, students could ask, ‘Do you guys look?' ” said Jim Bock, the dean of admissions at Swarthmore College in Swarthmore, Pa. “It’s a fair question."

While applicants may submit online material, like links to an app or video they created, Mr. Bock prohibits admissions officers from going online on their own to look for information about candidates. “Some colleges look; some do not,” he said. “As consumers, students should have access to that information.”

But some admissions directors who have a general policy not to seek out online material about applicants have occasionally made exceptions under special circumstances.

At Reed College in Portland, Ore., in the mid-2000s, for instance, admissions officers were alerted to an online chat room where an applicant who had figured out a loophole in the financial aid system was encouraging others to take advantage of it.

“It did factor into the admissions decision,” said Paul Marthers, who was the dean of admissions at Reed at the time and is now the vice provost for enrollment management and student success at the State University of New York system. “We considered it to be a financial aid fraud situation.”