The mayor 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 as children were leaving the school for the lunch break.
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' 7th district.
"The doors are closed and secure. I know that you really need to introduce yourself at the entrance so to get in and collect the children. You can't just go in as you wish," said one mother, who gave her name only as Lucie.
The school, which enrolls children from nursery school to high school, had no comment.