Witnesses said a bomb was thrown over the school wall as the children were playing.
Benghazi, cradle of the 2011 NATO-backed uprising that toppled Moamer Kadhafi, has seen near-daily attacks on security and other targets in recent months as the weak central government has tried to rein in former rebel brigades turned militias.
Fadia al-Barghathi, spokeswoman for Benghazi's Al-Jala hospital, had today said six children suffering "light to moderate" injuries were admitted there.
A security official described the force of the explosion as "weak" and said a hunt was under way for those behind the attack.
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"Witnesses saw an individual throw an explosive device over the school wall during recess," the official told AFP, speaking on condition of anonymity.
School blast today came after a patrol of Libya's Al-Saiqa special forces unit was attacked by unknown gunmen during the night in the centre of Benghazi, causing no injuries.
A security official said the armed group had then set upon a sit-in being staged by a group of anti-government protesters, torching their tents and cars.
The latest attacks on Al-Saiqa come after three members of the special forces were killed in a new wave of violence that struck Benghazi last week.
Clashes between militia made up of former rebels and the special forces erupted in the city after the son of Al-Saiqa's commander in Benghazi was kidnapped late January.
A military source said the abduction of Ali Abu Khamada, son of military commander Wanis Abu Khamada, was aimed at pressuring the special forces to bring about the release of prisoners held by the army.