The girl had tripped over a rudimentary bomb that was placed outside her school and it exploded once she picked it up, a police official said.
Egypt has witnessed sporadic bombings since the army ousted Islamist president Mohamed Morsi in 2013, mainly targeting security forces but which have killed several civilians in recent weeks across the country.
Last week, two people were killed when assailants threw a bomb at the entrance of a youth leisure centre on the outskirts of Cairo.
More sophisticated attacks have killed scores of security personnel mainly in the restive Sinai Peninsula, where security forces are battling an Islamist insurgency spearheaded by the Egyptian branch of the Islamic State group, Ansar Beit al-Maqdis.
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The group says its attacks only target security personnel in retaliation for a brutal government crackdown that has left hundreds dead and thousands jailed.
The interior ministry said meanwhile that police had arrested two men suspected in an attack on Wednesday that left two police officers wounded.
Gunmen on speeding motorbikes fired at the police officers as they guarded the Congolese embassy situated in a leafy Cairo suburb.