The difficulties of learning in a pandemic are taking a big academic toll on students in Palm Beach County’s public schools, with the worst setbacks coming in poor communities where more children learn from home. The number of students failing a class has doubled this year in the school district’s middle and high schools, while the total number of F grades nearly tripled, records from the first grading period show. School administrators blame the spike in failing students squarely on the challenges of online learning, where it is easy for students to tune out and some children struggle to get online at all.