Chelsea Watrous Cook, 32, was booked into jail on one count of aggravated murder after the shooting on Sunday.
Cook taught health and yoga at Skyridge High School in Lehi, according to the Alpine School District.
According to court documents, police were called to an apartment around 7 p.m. When they arrived, they found Cook's ex-husband pinning her against the wall.
Lisa Vilate Williams, 26, is dating Cook's ex-husband. She was found at the apartment with two gunshot wounds in her torso. She later died from her injuries.
Police learned Cook had come to the apartment to deliver cold medicine to one of their children.
Cook remained in the apartment unlawfully after being asked to leave, and she locked herself in a bathroom.
She eventually left the bathroom, but then she suddenly pulled out a handgun and fired three to five rounds at Williams.
Williams fell to the couch after being shot.
Cook's ex-husband took the gun away from her and tried to help Williams. When Cook tried to leave, her ex-husband pinned her against the wall until police arrived.
He told police their 3-year-old twins were present during the ordeal.
Skyridge High School sent a letter to students and parents Monday following the arrest of Cook.
The letter states in part:
"This morning we learned about a situation involving one of our Skyridge teachers, Ms. Cook. She was arrested and charged with a serious crime. We know there will be information in the media regarding this incident. We want you to be aware of what we have learned, so we can help you process and provide support to any of you who may be in need.
This type of news is hard to comprehend and we want you to know that teachers, counselors and your parents can help provide support in this difficult time."