KHARKIV, Ukraine — It had been more than 2？ years since Polina Zamorska last set foot in a classroom, but she was determined to return for her final year at School 80.

       To do so, she’d have to go underground.

       On Monday, the 16-year-old donned a traditional white smock with colorful embroidery and headed with her mom to Ukraine’s first permanent subterranean school, where a celebration with the oldest and youngest students — who would rotate in shifts through the space each week — was planned.