The exempt group included 27 students, teachers and family members, the state government said in a statement during the abduction.
The gunmen, who were reportedly carrying military supplies, stormed Kagara Government Science Secondary School on February 17. A student who died during the attack is named Benjamin Habila.
The executive governor of the state of Niger, Abubakar Sani Bello, tweeted that the released were received by the state government.
It comes as hundreds of schoolgirls were abducted in the early hours of the morning when gunmen stormed a state-run school in the state of Zamfara, northwestern Nigeria.
The schoolgirls were taken away from their residences by armed men who raided the Government Girls Secondary School in the city of Jangebe, a senior government official with knowledge of the incident told CNN.