Albert Coleman, 59, is accused of slipping a string around the child's neck and drawing it tight, police said.
He was fired by the Newark school district, and police charged him with aggravated assault following Monday's incident at the Elliott Street School.
A Newark Public Schools spokeswoman said the incident happened to the second-grader at an after-school program and said the teacher was terminated immediately.
Authorities said Coleman reportedly told the boy that he was going to strangle him if he didn't do his homework. When the boy didn't, police said, Coleman had the boy stand on a chair.
He then took the string attached to the ceiling light and tightened it around the boy's neck and released it, authorities said.
Police said the boy left the classroom and was seen by a school secretary who put water on his bruised neck.