Media reports say the youngsters aged seven to nine were playing with two mattresses at their school when some became trapped underneath.
That led to others rushing into a stairwell, where several fell and were crushed.
Following the stampede, Kunming officials demanded all schools conduct security checks to remove hazards.
The injured students have been taken to hospital and two were in critical condition, state media Xinhua reported.
Six children have been killed and 22 injured in a stampede at a school in the southern Chinese city of Kunming.