Lady Reading Hospital spokesman, Mohammad Asim, told CNN there were children among the injured.
The school is located on Peshawar’s ring road, on the southern outskirts of the city.
Peshawar’s superintendent of police Waqar Azeem told CNN the blast occurred during the morning’s first lecture. Rescue services and a bomb disposal squad are on site.
Peshar Police Chief Mohammad Ali Gandapur told CNN the cause of the explosion was an IED with 5-6 kilograms of explosives used in an abandoned bag at the religious seminary Jamia Zubairia madrassa.
This is the deadliest attack on civilians this year since the Pakistan stock exchange shooting in Karachi in June.
On Twitter, Pakistan Prime Minister Imran Khan said he was “deeply saddened” by the news.
“My condolences go to the victims families & prayers for early recovery of the injured. I want to assure my nation we will ensure the terrorists responsible for this cowardly barbaric attack are brought to justice asap,” Khan said.