A defence ministry official said: "The government officially announced that the August vacation will be advanced to Tuesday instead of next week.
"Parents have expressed concern that there could be retaliatory strikes."
Tamil Tigers said that 61 girls were killed and 150 wounded in the bombing inside Tiger-held Mullaitivu district on Monday.
The government said it had targeted a Liberation Tigers of Tamil Eelam (LTTE) training centre and that those killed could have been child soldiers recruited by the separatists.
A team from the United Nations children's fund visited the site and said they found no evidence to support the government's claims.
Joanna van Gerten, Unicef's Sri Lankan representative, said the team was "horrified" by the carnage.
"These were children from surrounding schools in the area who were brought there for a two-day training workshop on first aid, by whom we don't know yet," she said.
Van Gerten said the Unicef team found more than 100 children, mostly girls aged between 16 to 19, being treated for various injuries. "Most of the children we saw in the hospital had head injuries, shrapnel injuries," she added.
The LTTE said it was observing a day of mourning for the children on Tuesday.
In Colombo, police have put in place new parking regulations after the convoy of Pakistan's high commissioner was hit using a Claymore mine rigged to a parked three-wheel taxi.
A police spokesman said: "We have several new measures in place from today. In fact, when the bomb went off on Monday, senior inspectors were having a meeting about these new traffic arrangements."
The ambassador escaped unhurt, but four of his bodyguards and three civilians were killed.
The attack was the first on a foreign diplomat in Sri Lanka during more than 20 years of violence. Defence ministry officials said they were reviewing security for VIP visits after the attack.
The Tigers and the government both insist they are still observing a ceasefire agreed in 2002 but that the other is breaking it.