The teen has since been upgraded to good condition.
The fight apparently started between two students inside Hirsch Metro High School in the city's Grand Crossing neighborhood.
Students say they are shocked and confused over what took place. They say everyone inside is talking about it.
Senior Najah Henderson and her fellow classmates described what happened inside Hirsch High School during second period.
"They locked down the hallways," Henderson said. "If you were in the hallways, they told you to go to class, no walking around, it's very chaotic."
Students tell ABC 7 that two teenage girls, one of them pregnant, began fighting. Then things got heated.
"I was like, 'She stabbed her?' Why didn't anyone jump in and grab the scissors?" said Henderson.
Chicago Police say the 15-year-old student was stabbed in the neck with a pair of scissors inside a classroom.
It is not clear if the suspect brought in the scissors or grabbed them from inside the class. The school does have metal detectors. What is clear is that this incident caused chaos inside and outside the school.
"They were trying to rush everyone away. They were trying to get the other girl arrested and the other girl medical attention," said senior Brandon Shelvin.
The pregnant girl was rushed to the hospital in critical condition and was later upgraded to good condition.
Security is present inside the school. Officials did not want to comment about what happened, but they are trying to get to the bottom of it.
"They are just trying to find out what happened," said Shelvin. "They are talking to everybody that heard...just trying to find out what happened."
The students do admit this school does have its share of problems. Even former student Dominique Crumbley doesn't find this incident that hard to believe.
"When I used to go here, it was so many fights, so much everything going on, so it doesn't surprise me that it's a stabbing," said Dominique Crumbley, who graduated in 2010.
One student told ABC 7 there may have been more people involved. Police did take a female student into custody. No word yet on charges.