A 12-year-old boy was charged Wednesday with stabbing his 65-year-old teacher in the back as she sat at a desk correcting papers, authorities said.

Police said the teacher suffered a 2-inch-deep knife wound requiring 20 stitches in the stabbing that occurred Tuesday while the boy, a student at Ford Elementary School in East Los Angeles, was completing a tardy assignment.

School security officials later arrested the student on the playground, where he was sitting holding the knife, investigators said.