The rebel Liberation Tigers of Tamil Eelam (LTTE) said 51 out of 55 people killed in Monday's air raid were children.
The guerrillas had earlier said that 61 schoolgirls perished in the bombing.
There was no immediate explanation for the discrepancy in the figures, but the guerrillas listed children from 11 schools as killed in the bombing.
The Sri Lankan government has admitted the attack, but denied killing children.
The defence ministry has maintained it targeted an LTTE military training centre.
However, both UNICEF and the independent truce monitors have said they found no evidence to support government claims that the victims were combatants or were undergoing military training at the facility.
UNICEF and the Sri Lanka Monitoring Mission have also said they were shown only the bodies of 19 people, but that they had visited a large number of children wounded in the attack that involved four war planes.
The bombing came as government forces and Tamil Tiger rebe