Boko Haram claimed responsibility for last Friday’s abduction of the students from the all-boys school in the Katsina State village of Kankara.
When armed patrols go by, parents outside the school momentarily gain hope that they may have found their sons.
The girls had been kidnapped by the militant group a month ago.
The country's president called the incident a "national disaster."
The blast bears the hallmarks of a faction led by the group Abubakar Shekau, which forces women and girls to carry out suicide bombings.
Ngozi, a Nigerian teenage superheroine, is Marvel’s latest character. Her story will be the first to be set in a real-life African country and star a Nigerian superhero.
The Islamist militant group has killed more than 380 civilians in Nigeria and Cameroon since April.
Joy Bishara and Lydia Pogu detailed the horrors they faced when they were kidnapped.
The attack on Nigeria’s northeastern city of Maiduguri on Wednesday night killed at least 14 people.
The girls were among more than 200 kidnapped in April 2014.