Police spokesman, Emmanuel Ojukwu, told the BBC Hausa service the bombing had left 47 people dead and another 79 wounded.
Victims were rushed to a nearby hospital, said a teacher, who asked not to be identified.
One student told the BBC he saw mutilated bodies of fellow students at the scene. A resident reported seeing parents wailing at the sight of their children’s bodies at the hospital.
No one immediately claimed responsibility for the attack in Yobe State, a territory close to the stronghold of Boko Haram militants, who have staged a five-year insurgency.
“So far, the number of the dead is 48, while 79 are injured. I counted the bodies, mostly students and a few teachers,” a nurse at Potiskum General Hospital told Reuters.
“A teacher who survived the blast with minor injury said the bomber dressed like a student and was also on the assembly ground with the students,” she said, asking to remain anonymous.
Mariam Ibrahim, a teacher at the Government Science Secondary School (GSS) in Potiskum told Reuters the bomb went off as she was arriving and students were at morning assembly.
Potiskum resident Aliyu Abubakar said he heard the explosion when he was dropping off his two sons at a nearby Islamic college. “One of my sons fell down, I came out dragged him in and we drove off back home,” he said.
A second teacher, asking to remain anonymous, said, “There are some (others) that are critically injured and I am sure the death toll will rise.”
The school was a mass of abandoned footwear and blood, resident Adamu Alkassim told AFP news agency.