She was taken to Östra Hospital with the knife still in her throat, according to a statement from police, and was later moved to Sahlgrenska Hospital.
Emergency services received a call about the stabbing at 9.51am on Monday morning.
"The patient was loaded into the ambulance at twelve minutes past ten," Jack Söderberg, a shift leader with the emergency service operator SOS Alarm, told the TT news agency.
According to police spokesperson Björ Blixter, the girl is conscious and told police she had never seen her attacker before.
“We've been able to talk to her. There's nothing to indicate that the injuries are life threatening, but she's still seriously wounded,” he told the Expressen newspaper.
Police are also speaking to family members and acquaintances in hopes of coming up with a possible motive for the attack.
“We don't have any motive at the moment,” Blixter told the Dagens Nyheter (DN) newspaper.
Later on Monday, police hinted that they couldn't rule out that the incident may have been the result of some sort of accident.
Police spokesperson Mats Haake told the TT news agency there were conflicting witness reports about what happened and that it was “unclear” whether the stabbing was indeed a crime.
“We're trying to collect information in order to figure things out,” he said.
According to police, there are no known threats against the girl or her family.
The perpetrator is believed to be a male in his twenties and remains at large.
He may have also been caught on film.
“There are surveillance cameras in the area and we're now gathering footage from them,” Blixter told DN.
The area where the attack occurred has been cordoned off pending a forensic investigation.
Police have classified the incident, which was witnessed by several people, as attempted manslaughter.
Following the attack, shaken parents and students gathered at the school.
“This is a quiet area,” one parent told the TT news agency.
The school sits between two six-storey apartment buildings and a wooded area in Hjällbo.
Monday's stabbing is the latest in a string of recent knife attacks at schools in Sweden.
In mid-December, 1 19-year-old was seriously injured after being stabbed several times in connection with a fight at the Österänggymnasiet in Kristianstad in southern Sweden. Two men were arrested following the incident.
A few weeks earlier, a 17-year-old student was cut with a knife while entering a high school in Vallentuna in suburban Stockholm.
The boy was seriously injured in the attack and remained sedated for several days.