The attack happened at the 1,200 pupil Maximilien-Perret school in Alfortville when two older students started arguing at around 10.15am.
One teacher told Le Parisien newspaper how staff tried to stop the fight.
"With my colleague, we tried to separate them," said Guillaume Lombardeau, a maths teacher.
Lombardeau explained that one of the pupils got out a knife and "in the struggle, my colleague got hurt."
The injured teacher was taken to a local hospital where his condition was not believed to be life threatening.
One of the boys was held by police shortly after the incident, while the other fled the scene and is still being hunted.
A union representative told Le Parisien that the attack took place in circumstances that are "more and more difficult and tense because there is less and less time spent with the pupils."
Additional security has been sent to the school, which will reopen on Wednesday.