At least 25 people were killed yesterday when militants blew up the bus in the capital of restive Baluchistan province and then stormed a hospital where survivors had been taken for treatment.
"This was a cowardly and desperate attempt to deny girls their right to education," Malala, 15, said in a statement.
Former British prime minister Gordon Brown, the United Nations special envoy for global education, said it was the "bloodiest atrocity yet in escalating violence against female students".
She was flown to Britain for surgery on her head injuries and returned to school in Birmingham, central England, in March.
Malala has become a global symbol of the campaign for the right of girls to an education and has been nominated for the Nobel Peace Prize.