Tamil Tiger rebels in Sri Lanka have admitted kidnapping at least 21 students in the east of the country.
The Tigers have routinely denied recruiting underage fighters
A rebel spokesman told the BBC that junior Tiger fighters had made a "serious mistake" in taking the children from their school.
Correspondents say that in the past the Tigers have always denied abducting children to use them as soldiers.
This year has seen a sharp increase in violence in Sri Lanka, with at least 3,400 people dead, the authorities say.
Rebel military spokesman Rasaiah Ilanthirayan told the BBC Sinhala service that the junior Tigers who abducted the children had been expelled from the movement.
"It was a mistake by the field commanders, but we are taking strict disciplinary action against our own people involved in this," he said.
Mr Ilanthirayan said that the children had been freed and were being handed back to their parents.
In a statement issued later on Tuesday, Mr Ilanthirayan said that the teenaged pupils wanted to join the rebel movement and had come of their own volition to Tiger-held areas.
The statement said that when it was discovered they were underage, they were told to return home.
Army spokesman Brig Prasad Samarasinghe told the BBC News website that the children, mostly teenage girls, were abducted in the eastern Ampara area.
Brig Samarasinghe said 21 girls and three boys, all teenagers, were preparing for a school examination when they were picked up by the rebels on Monday evening.
"The incident was reported to the local police. The police have informed the peace monitors and the human rights commission," he said.
Brig Samarasinghe said the children could have been taken away for "forced conscription".
Both sides are accused of preparing for all out war
The BBC's Roland Buerk in Colombo said that the incident has once again highlighted the use of children as combatants in Sri Lanka.
Our correspondent says that all sides are believed to be building up their forces, preparing for a possible return to full scale war.
The Tigers have in the past denied recruiting child troops.
But they have been accused repeatedly of doing so by human rights groups and the UN throughout Sri Lanka's years of conflict.
The leader of a breakaway faction of the Tigers now, Colonel Karuna, has also been accused of recruiting child soldiers.
Last month, the United Nations children's agency, Unicef, said that "elements in the Sri Lankan military" were helping Col Karuna to abduct children to fight the Tamil Tigers.
A senior UN official said there was "credible evidence" that troops had rounded up children to fight with the renegade rebel group.
The authorities rejected the allegations,
The Tamil Tigers have been fighting for independence for the 2.5m-strong minority Tamil community in the north and east of the country.