A group of armed men attacked Nawaeb Lutheran Secondary School on Thursday night.
They entered the female dormitory and held the students at gunpoint.
A Grade 10 student said the girls had just finished their exam and were inside the dormitory.
She said the men stole their phones and started beating them with sticks and knives.
The attackers locked the dormitory doors, and the girls screamed for help after after which male students and staff responded and chased the men away.
Eight injured female students were taken to Angau Hospital and a male student was also hurt while trying to help.
Dr. Kipas Binga from the Morobe Provincial Health Authority said one girl had wounds to her back and shoulder.
Another had a cut to her ear. Others had injuries to their arms and legs.
One girl had a skull fracture but is now stable and waiting for surgery.
Nawaeb MP Theo Pelgen condemned the attack and said those responsible will be punished.
Morobe Police Commander Jacob Singura said the incident happened between 7pm and 8pm. Police are investigating and trying to identify the suspects.
Singura called on communities to respect schools and students.
He said such attacks are not acceptable.
Two police vehicles visited the school on Friday to collect statements.
This is the first time such an incident has happened at the school.