Taliban militants stormed the Army Public School in the north-western city of Peshawar on 16 December 2014.
The hangings come just two weeks ahead of the anniversary of the attack, which shocked the nation.
The massacre prompted a crackdown on Islamist militants, the establishment of military courts to try terror suspects and the resumption of capital punishment after a six-year moratorium.
Earlier this week, Army Chief General Raheel Sharif signed death warrants for the four militants who were named as Maulvi Abdus Salam, Hazrat Ali, Mujeebur Rehman and Sabeel.
`Shooting indiscriminately`
Security officials said the four men were executed early on Wednesday at a jail in the north-western city of Kohat.
They were convicted in a military court of aiding and abetting the militants who carried out the attack, all of whom were killed in battle with security forces on the day.
The four have been identified as part of the Toheedwal Jihad Group, a little-known faction of the Pakistani Taliban.
During the assault, militants scaled the walls to get into the school and set off a bomb before moving from classroom to classroom shooting indiscriminately at both children and teachers.
The school was near a military complex in Peshawar and a number of its students were the children of military personnel.
"The rest should be caught too, no one should be spared," Waheed Anjum, an 18-year-old survivor of the attack, told AFP news agency.
"The hangings won`t bring back my son, but now other people`s sons will be kept safer," said Tufail Ahmed Khan, who lost one son in the attack while another was wounded.
Peshawar, which is close to the Afghan border, has seen some of the worst of the militant violence to assail the country over the last decade.
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