President Francois Hollande "joins the UN secretary general (Ban Ki-moon) in considering the act 'unjustifiable'," the president's office said in a statement.
Paris "demands the establishment of an immediate ceasefire," the statement said, adding that "all efforts must converge on this goal."
The White House also condemned the shelling of the school in the Jabaliya refugee camp in northern Gaza, where more than 3,000 displaced Palestinians were sheltering from an assault by Israeli forces on Hamas militants.
It is the second time in a week that a UN school housing refugees has been hit.
Israel had said its truce, which began at 1200 GMT, would not apply in places were troops were "currently operating", hours after the army made what it called a "significant advance" into the narrow coastal strip.