Hassan Abdul-Bashir was in bed in his dorm in Kankara, Nigeria when the gunshots started on Friday night. The 13-year-old was ordered outside, where gunmen were ushering everyone towards the school's gates. That's when he decided to run. He took cover under a faulty school bus, then ran to the school's fence and hid there.

He escaped. But an unknown number of students from the Government Science Secondary School Kankara, including Abdul-Bashir's cousin and friend, remain missing after gunmen armed with AK-47 rifles attacked the school on Friday.

"They commanded the crowd like a herdsman herd the sheep," Abdul-Bashir told CNN on Saturday. He said the gunmen were asking students for money, ransacking their lockers and taking some of their belongings. "They shot the policeman guarding our school. I saw them driving many students. There could (be) as much as 200 students, but I am not sure," he said.

Local police told CNN a large number of attackers riding motorbikes ambushed the boys' secondary school in a possible kidnapping-for-ransom attempt.

Katsina police spokesperson Isa Gambo said in a statement that reinforcements arriving to the scene "forced the hoodlums to retreat back into the forest."

Speaking to CNN on Saturday, Gambo said 200 students had been safely returned to the school by Saturday morning, but stressed it was "too early" to know "how many students are missing or if some of them were kidnapped." The state's educational commissioner Lawal Badmasi said some of the students were still believed to be with the abductors.

It is unclear how many people were at the school at the time of the attack, as some might have gone home after exams, the authorities said. The army was visiting the homes of students' families to find out who is still missing or may have been abducted, Kastina's Director-General on Media Abdu Labaran told CNN.

Badmasi said 436 students had been accounted for. He said similar government secondary schools in the state normally have about 700 students. However, speaking to CNN, two students from the school estimated that more than 1,200 attend the school, taking into the account the absence of the students from the school's most senior class, who have left after finishing their final year exams.

Nigerian President Muhammadu Buhari condemned the "bandits' attack" and ordered the army and police to pursue the attackers and "ensure that no students are harmed or go missing," Buhari's assistant Garba Shehu told CNN on Saturday.

According to a report sent to the President by local Governor Aminu Bello Masari and the army, the military exchanged fire with the gunmen in an ongoing operation at their enclave at Zango/Paula forest in Kankara on Saturday. At that time, there were no reports of student casualties, according to the report.

"The police, Nigerian Army, and Nigerian Air Force are working closely with the school authorities to ascertain the actual number of the missing and/or kidnapped students while search parties are assiduously working with a view to find and/or rescue the missing students," Gambo said in the statement. He added that extra security had been provided to the school.

Musa Adamu, a senior secondary student at the school, said he and many of his roommates jumped out of the windows when they heard the gunshots on Friday. "We headed to the fence and climbed on it and jumped down," the 18-year-old told CNN. "The gun shots sound got louder, we ran in different directions ... into the forest. Most of us had no shoes on, and we kept running until we got tired and the sound of the gunshots fainted."

Adamu said he spent the night in the forest with about 20 other students. It wasn't until the next morning when they realized they had injured their legs and feet during the escape. They walked back into the town, meeting other students hiding in the forest on the way. "When we got to school, we saw soldiers, and they asked us to come in and write our names, after a while they told us to go pack our things and go home," he said.

Nigerian authorities said the motive for the attack is unclear, but added the area has seen kidnappings for ransom attacks in the past.

Abdul-Bashir said he believed the attackers were Fulanis, members of the traditionally nomadic herder ethnicity. CNN was unable to independently verify that claim.

There have been numerous violent clashes between the Fulanis and Christian farmers in north-western Nigeria, according to the International Crisis Group. The group said the violence has killed more than 8,000 people since 2011, and forced over 200,000 to flee their homes.