Students and teachers died in the attack, which triggered a gunbattle that lasted for about six hours.
Lt Gen Asim Salim Bajwa said those arrested had provided the attackers with shelter, transport and weapons.
Another three suspects, including two women, were still at large, he said.
A Taliban splinter group claimed responsibility and has threatened similar attacks. The main Taliban organisation denied any involvement.
The assault came 14 months after gunmen affiliated with the Pakistani militant group Tehreek-e-Taliban attacked an army school in Peshawar, killing 132 children.
Since then, Pakistan has killed and arrested hundreds of suspected militants under a counter-terrorism plan enacted in the wake of the massacre.
Pakistan’s prime minister, Nawaz Sharif, said after the attack: “We are determined and resolved in our commitment to wipe out the menace of terrorism from our homeland.”
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