Kids wear masks as they return to school in Hillsborough County in August of 2021. PHILADELPHIA - Pennsylvania's largest teachers union on Wednesday urged K-12 schools to require masks in school buildings, a measure that state officials are encouraging but not mandating.The Pennsylvania State Education Association cited the threat of the highly contagious delta variant of the coronavirus, which is spiking infections and hospitalizations across the state and nationally, including among children.The U.S.