The kidnapping in Kuriga, Kaduna state on March 7 was one of the biggest such attacks in years and prompted a national outcry over insecurity.
Teachers and residents previously said around 280 pupils were kidnapped, but the army said 137 pupils were freed.
"The rescued hostages totalling 137 comprise of 76 females and 61 males. They were rescued in Zamfara State and would be conveyed and handed over to the Kaduna State Government for further action," said army spokesman Major General Edward Buba.
He told AFP the number represented all the pupils who were in captivity. The numbers of those reported abducted in Nigeria are often lowered after people who went missing while fleeing attacks return home.
Press reports said the pupils were aged between eight and 15.
"The abducted Kuriga school children are released unharmed," Kaduna state governor Uba Sani said in a statement that did not specify how they were freed.
"This is indeed a day of joy," he said, thanking the army, President Bola Ahmed Tinubu, the national security adviser and "all Nigerians who prayed fervently for the safe return of the school children."
FILE—Nigerian army patrol near LEA Primary and Secondary School Kuriga where students were kidnapped in Kuriga, Kaduna, Nigeria, on March 9, 2024.
Gangs of criminals known locally as bandits have been blamed for the abductions. They routinely target communities, loot villages and carry out mass kidnappings for ransom in northwest and north-central Nigeria.
Officials said troops had been searching forests to rescue the pupils, though few details had emerged.
Relatives had said the kidnappers demanded a large payment for the return of the children, but President Tinubu said he had ordered security forces not to pay up.
Kidnap victims in Nigeria are often freed following negotiations with the authorities. A 2022 law banned handing over money to kidnappers and officials deny ransom payments are made.
Wave of abductions
Nigeria has been hit by a wave of mass kidnappings and many victims are still missing.
Last weekend, kidnappers abducted more than 100 people in two attacks in Kaduna state.
On Saturday, the army said it had rescued 16 pupils kidnapped just days after the Kuriga attack from a school in Sokoto, also in the northwest.
In the early 2000s, kidnappers targeted oil workers in the Niger Delta, but hostage-taking has since spiraled into a nationwide industry and has become a favoured tactic of bandit gangs and jihadists.
Nigerian risk consultancy SBM Intelligence said it had recorded 4,777 people abducted since Tinubu took office in May last year.
Some experts believe the country's economic crisis is now driving a rise in kidnappings as desperate Nigerians turn to crime for income.
The mass kidnapping in Kaduna State and another in the northeast came almost 10 years after Boko Haram militants triggered a huge international outcry in 2014 by abducting more than 250 schoolgirls from Chibok in the northeast.
Boko Haram and rival group Islamic State West Africa Province (ISWAP) still regularly carry out abductions in the northeast.
But with the rise of heavily armed gangs, the northwest has also become severely affected by kidnappings.
The gangs have targeted schools and colleges in the past, but there had been a lull in these attacks before the Kuriga abductions.