Open this photo in gallery: David Falconer, founder and executive director of Classrooms Without Walls, on Feb. 7 in his home office, which also serves as his virtual classroom.Sarah B Groot/The Globe and Mail Once a week, Sarah Marshall, a high-school biology teacher who lives just north of Nanaimo, B.C., would log on to Zoom for a class unlike any she had taught before. Her students ranged in age from 7 to 16. Almost all of them were in Ukraine, although some joined her virtual classroom from Turkey and other places they had been sent to live after Russia invaded…