At least 20 students have reportedly been killed by a bomb at a school assembly in the north-east Nigerian town of Potiskum.
The explosion happened at a boys' science and technical school in the town.
Militant group Boko Haram is likely to be blamed for the attack.
It has often targeted schools during its five-year campaign to establish an Islamic state, which has left thousands dead.
"The students had gathered for the morning assembly when something exploded in their midst with a thunderous sound at exactly 7:50 am [06:50 GMT]," a sobbing teacher told AFP news agency.
Victims were being rushed to a nearby hospital, said the teacher, who asked not to be identified.
A witness told the BBC he saw parents wailing at the sight of their children's bodies at the hospital.
The school was a mass of abandoned footwear and blood, resident Adamu Alkassim told AFP.
The blast underlines the lack of security in north-eastern Nigeria, says BBC Nigeria correspondent Will Ross reports.
Boko Haram, whose names means Western education is forbidden, is waging a sustained campaign to prevent children from going to school.
It believes girls should not attend school and boys should only receive an Islamic education.
In April, the group sparked global outrage by abducting more than 200 girls from a boarding school in Chibok town in Borno state.
It says the children have converted to Islam, are learning to memorise the Koran and have been married off.
Potiskum in Yobe state has often been targeted by Boko Haram.
Last week, a suicide bombing killed 15 people in the town.
The bomber joined a religious procession of the rival Shia Muslim sect, before blowing himself.