More than 500 cases at the University of Alabama in Tuscaloosa. Nearly 160 at the University of Missouri in Columbia.

Colleges and universities that brought students back to campus are expressing alarm about coronavirus infections emerging as classes have barely started, raising the possibility everyone could be sent home.

“The rise we’ve seen in recent days is unacceptable, and if unchecked, threatens our ability to complete the rest of the semester on campus,” University of Alabama President Stuart Bell said at a news conference on Monday, five days after classes resumed, as the mayor of Tuscaloosa temporarily closed bars and warned that the local health system could become overwhelmed.

While a growing number of schools have backed off reopening, opting instead for online classes, others are hoping a host of new rules and adaptations can keep the coronavirus at bay. They are requiring masks, mandating testing and threatening students and campus groups with penalties for partying. Ohio State University said this week that it had suspended 228 students for virus-related violations.

Despite the precautions, schools are quickly discovering large outbreaks that have prompted new soul-searching about students’ commitment to social distancing and universities’ ability to deliver even a dramatically transformed on-campus experience. Speaking Monday, Bell declined to blame young people’s lack of caution and emphasized that it is up to the university to work with everyone to minimize infections.

“Our challenge is not the students,” Bell said, though he acknowledged that multiple students are facing discipline for breaking coronavirus rules. “Our challenge is the virus.”

Tuscaloosa Mayor Walt Maddox, who joined Bell at the Monday news conference, said the university had requested his executive order closing bars for two weeks and eliminating bar service at restaurants. He said “the ever-increasing number of coronavirus cases on campus will create two major disruptions for Tuscaloosa if left unabated” — stretching the city’s health system and threatening its economy, which includes thousands of university jobs.

Among students, faculty and staff tested across the University of Alabama System, 566 have had positive results for the virus since Aug. 19, according to a dashboard that is being updated. Those numbers do not include “entry testing” conducted to make sure students leave home coronavirus-free.

The overwhelming majority of the new cases, 531, are at the flagship campus in Tuscaloosa.

“The truth is that fall in Tuscaloosa is in serious jeopardy,” Maddox said.

Other officials are striking similar notes of dismay at early case tallies — even on campuses where classes are completely online. Sarah Van Orman, the chief health officer for student health at the University of Southern California, wrote Monday that there had been “an alarming increase in the number of covid-19 cases in students in the University Park Campus community” during the first week of the semester.

In seven days, Van Orman said, 43 cases were identified, all of them “related to students in off-campus living environments.” More than 100 students are in 14-day quarantine because of coronavirus exposure, she said.

The school had originally planned to hold most classes virtually. But with no green-light for in-person instruction from local and state officials, it switched to all remote learning, and USC officials urged students not to return.

“Los Angeles is at a critical juncture in public health,” Van Orman wrote to students. “While no students have been hospitalized to this point, we all need to work together to protect those in our community who may be at higher risk of severe disease and prevent serious health outcomes for all.”

Iowa State University has reported 130 cases in the week since courses started. The University of Missouri described a “new normal” as campus reopened — from thinned-out lecture halls to an educational campaign from student leaders and influencers — but found itself with 159 coronavirus cases among students by the first day of class.

Some schools have reversed course as clusters have gotten out of hand. On Aug. 17, the University of North Carolina at Chapel Hill announced that it would revert to all-remote instruction for undergraduates after 177 coronavirus cases were confirmed among students and hundreds more went into quarantine because of possible exposure. The University of Notre Dame followed a day later, halting in-person teaching for at least two weeks in an attempt to contain infections.

Cases are still mounting, however. UNC-Chapel Hill says it received results from more than 1,500 tests last week, more than 500 of them positive. At least 784 students have tested positive to date, mostly after the start of the semester.

Schools pushing ahead with in-person learning are trying to show students that they are serious about social distancing.

Coronavirus clusters at Central Michigan University spurred concerns about parties, causing President Robert O. Davies to announce Monday the suspension of Greek organizations’ “in-person activities.” The school counted 54 new cases in seven days, Davies said.

Dozens of professors in Michigan want the governor to step in. More than 200 faculty members sent a letter to Gov. Gretchen Whitmer (D) urging that she “mandate that classes that don’t require face-to-face teaching as part of their pedagogy … must meet fully online.” The professors are mostly from Grand Valley State University but include faculty from Central Michigan University, Western Michigan University, the University of Michigan at Flint and Northern Michigan University, according to MLive.com.

Syracuse University published a blistering letter Aug. 20 saying that a “large group of first-year students” would face discipline for an evening quad event that “selfishly jeopardized” the residential experience. The letter warned that they “may have done damage enough to shut down campus, including residence halls and in-person learning, before the academic semester even begins.”

At Ohio State, where officials have reported one of the biggest crackdowns to date in the form of suspensions, university spokesman Benjamin Johnson emphasized that the school has an enrollment of nearly 70,000 students, many of whom are following the rules.

“If you were to walk around campus with me right now … you would see hundreds, really thousands, of students doing everything right,” Johnson told The Washington Post.

But a few people’s decisions can reverberate.

“We are reminding students at every possible opportunity that each individual’s choices affect all of us here on campus, and so we all have to do the right thing if we are going to be able to remain here on campus,” Johnson said. “And if we don’t all do the right thing, we could have to go home.”

Antonia Farzan contributed to this report.

Correction: A previous version of this story suggested that University of Southern California has brought students back to campus. It is fully remote.

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