Footage on social media allegedly showed Simpkins getting into an altercation during class with another student that escalated to gunfire Wednesday.
The fight was broken up and the two students calmed down, but then Simpkins pulled out a firearm from his orange backpack and fired several shots that struck three people, according to an arrest warrant Thursday.
Calvin Pettitt, a 25-year-old English teacher who jumped in to break up the fight, was shot in the back, suffering broken ribs and a collapsed lung, according to police and his family.
Police have said a 15-year-old boy was in critical condition, while a teenage girl who was grazed by a bullet had been discharged from the hospital.
The fourth victim, a pregnant woman, was injured in a fall during the shooting, but received medical care on the scene, police said.
Simpkins was charged with three counts of aggravated assault with a deadly weapon