The footage raised fresh questions of a possible split in the fractured Taliban leadership, whose official spokesman has denied the group was behind the assault.
Militants scaled the walls of Bacha Khan University in Charsadda on Wednesday morning and killed 20 people before being gunned down by army commandos and police.
Pakistani Taliban spokesman Mohammad Khorasani issued a written statement that evening disassociating the group from the attack and calling it un-Islamic. But the same day, a Taliban faction commander Umar Mansoor told Reuters his fighters had targeted the campus because it prepared students to join the government and army.
Mansoor is considered close to Mullah Fazlullah, the embattled leader of the fractious Pakistan Taliban group. The reason for the conflicting claims by the official spokesman and Mansoor was not immediately clear but has led to speculation of a possible split in the Taliban leadership.
“Now we will not kill the soldier in his cantonment, the lawyer in the court or the politician in parliament but in the places where they are prepared, the schools, the universities, the colleges that lay their foundation,” Mansoor said in the video.
“With the mercy of god, our attacks on all universities and schools will continue.”
The video also shows four attackers, two of them in their mid-teens, practising shooting as part of their training before carrying out the Charsadda attack.
Pakistan has killed and arrested hundreds of suspected militants under a major crackdown launched after Taliban gunmen massacred 134 children at a military-run school in the northwestern city of Peshawar in December 2014.
The Peshawar school attack was seen as having hardened Pakistan’s resolve to fight militants along its border with Afghanistan.