Hundreds of thousands of Los Angeles Unified students are returning to campus from winter break Tuesday amid a record-breaking surge in coronavirus cases that is testing the carefully laid plans of educators in the nation’s second-largest school district to keep classrooms open amid stressful conditions.L.A. Unified officials are girding for staffing shortages, large numbers of student absences, and long lines to access campuses for the multitudes of students who will probably need coronavirus tests to meet the district’s requirement that a negative test be shown by anyone seeking to enter school grounds. Never has the coronavirus caseload in schools been higher. On Monday, district data showed a massive spike in active cases among students and staff to more than 58,000, with more than 760 schools reporting more than 10 cases, more than 140 reporting more than 100 cases and six high schools reporting more than 300, according to an LA Times database of district cases.About 15% of the more than 400,000 tests submitted between Jan. 3 and Jan. 9 were positive, according to the district’s data. The numbers are still in flux. About 82% of the district’s 450,000 students had uploaded their test results by Monday afternoon, meaning thousands could show up to school Tuesday morning still in need of a test. Schools are hoping to have enough rapid antigen tests available so that students can attend class, said Dr. Smita Malhotra, the district’s medical director. In cases where they are not available, students may be directed to the district’s sites for PCR tests, but results will take about 24 hours. District officials said the extensive safety protocols the district has adopted have prepared them to meet a moment that is exerting pressure on schools across the country as they work to keep students learning in person. L.A. Unified holds a unique position among school districts with the largest weekly coronavirus testing program in the nation, a massive effort that tests every student and staff member — more than 500,000 people — every week. The program is key to the school district’s ability to identify positive cases as a soon as possible to help fend off outbreaks.“Omicron is really testing the infrastructure and capacity of school districts,” said school board President Kelly Gonez. “We’re lucky to have a strong foundation in place to prepare us for this moment.”Some students and staff who tested positive early last week may still be able to return to campus Tuesday if it has been six days since the result and they have a negative test, among other requirements. Still, the large case numbers make it likely that schools — which were already grappling with teacher and substitute shortages — will face additional staffing shortfalls. To help fill the gap, the district is preparing to deploy about 4,000 administrators, certificated staff and other employees, officials said.In a video message Monday, LAUSD Supt. Megan K. Reilly acknowledged the concerns. “We know there is apprehension, and we’ve added the extra layers of protection for the return to school,” she said. ‘“We know that opening schools under these circumstances has bumps along the way.”In addition to the testing requirement, the district will continue to uphold previous safety measures, including masking indoors and outdoors and upgraded air filtration systems. About 90% of eligible students have been vaccinated, district officials said.In a statement, Cecily Myart-Cruz, president of the L.A. teachers union, said: “LAUSD is in a better position than most others in the country because of the safety infrastructure that educators and families fought for and won during this pandemic.“The terrain is changing by the day, and educators will continue to prioritize the health and safety of everyone in our education community,” she said. At San Pedro High School, Principal Steve Gebhart said Monday that he knew of about a dozen teachers out of 127 who would be out on the first day back due to coronavirus infections. Though they are preparing to have those classes covered by substitutes, it will be “essentially all hands on deck,” with teachers asked to cover classes during their planning periods. “The hope is that we keep it covered in a relatively traditional way,” Gebhart said. “I’m cautiously ready for it,” he said about students returning. “The unexpected is expected, so we’re pretty good at rolling with things by now.”Gebhart was also expecting a large number of students on his campus of 2,850 to show up still needing a coronavirus test — and they may miss the first day of school while awaiting results. Teachers will also have to be ready to pivot their lesson plans if a chunk of their classes are missing, Gebhart said. But with guidance geared toward emphasizing mastery in learning, rather than strict grading rules, he said he hopes teachers will feel less stressed at missed instructional days.At the Edward Roybal Learning Center, Principal Blanca Cruz said she also prepared teachers by coordinating backup instruction material in case a teacher has to isolate at home. Roybal’s librarian is leading an effort to create backup plans for different subjects at all class levels in case support staff have to step in as substitute teachers, Cruz said. “It’s taken all of us to come together to make sure our staff is ready,” she said. Supt. Frances Baez — who oversees the Central District, including schools in Eagle Rock, Los Feliz and South Los Angeles — spent Monday calling schools to confirm they had received rapid antigen coronavirus tests for Tuesday morning. Additional staff members will also be reporting to school sites tomorrow to cover shortages, she said.“We feel very ready for this,” Baez said. School board member Nick Melvoin said the next few weeks will probably be challenging. But he believes there is now widespread community acknowledgment that school closures were bad for kids and broad support for keeping campuses open. “The consensus that has emerged is that when bars and card rooms are open, schools need to be open,” he said. “That is a consensus that didn’t emerge during the first year of the pandemic.”Times data journalist Iris Lee contributed to this story.