"Schools that have temporarily closed classes."
"schools" 「学校」
"that have temporarily closed classes" 「学級閉鎖になった」
"schools that have temporarily closed" も言えます。
"Not just adults, but students and small children have been infected, some schools have even closed for classes."
「大人だけでなく、学生や小さな子供にも感染者が出ており、学級閉鎖になっている学校もある。」