President Joe Biden's pledge to reopen most American schools within his first 100 days in office is in danger of going off the rails as teachers' unions hold firm on their demands for new safety measures amid conflicting messages from the administration and public health leaders.

White House press secretary Jen Psaki on Thursday said Biden was committed "to ensuring schools are open five days a week" -- once the proper safety measures are in place. New guidelines from the US Centers for Disease Control and Prevention are expected on Friday, she added.

It was Psaki's third swing in 72 hours at questions over the President's definition of a successful return. Her initial suggestion, on Tuesday, that "teaching at least one day a week in the majority of schools by day 100" might be enough to clear the bar was met with a torrent of criticism from parents, teachers and administrators.

Nearly a year into the unprecedented national experiment in remote learning in the face of a growing pandemic, millions of children are still at home, with no expected return date. Studies show them falling behind, with low-income or Black and Latino children hurting the worst -- but the science is unclear on how, or when, they can safely return to classes, even as some teachers get vaccinated and schools districts work to fit classrooms for reopening.