The man, said to be in his 50s, battled his way into the school past staff before making a brief declaration and shooting himself as the children left their class for lunch.
Paris mayor Bertrand Delanoe said his thoughts were with “all the little Parisians who were present, their parents and all the educational community”.
Police said the suicide happened shortly before noon.
The man’s motive and identity was not immediately known.
“Nobody informed us, we found out just like that,” said Christelle Parade, who was standing outside the school with her son Alexandre.
“Yes, my schoolmates told me that they saw the body, some blood but I don’t really know anything,” said schoolboy Alexandre Parade.
Others expressed surprise that the man gained access to the private La Rochefoucauld school, in the heart of Paris’s 7th district.
“The doors are closed and secure. You need to introduce yourself at the entrance to get in,” said one mother, who gave her name only as Lucie.