The first time she witnessed a student's major tantrum — a 2-year-old hurling a toy stove filled with plastic pots and pans—Shanikia Johnson had just started as a teacher at Little Flowers Child Development Center in West Baltimore.She knew toddlers acted out. But the rage-filled reaction, triggered when Johnson wouldn't allow the boy to play with a toy, stunned the 22-year-old teacher. Then, time and time again, she saw other children throwing classroom furniture. Bookcases, chairs, tables—all were flung around the room.Some students bit classmates, leaving teeth marks on hands and cheeks; a few threatened to hurt staff members. Other...