According to WFAA:
Dallas Police Deputy Chief Gil Garza said in a news conference the officer was out of uniform and headed to a family function when he saw an altercation. A woman was stopped at an intersection when another car, driven by the suspect, pulled up alongside her. Words were exchanged between the two, then the suspect fired multiple shots into the victim's vehicle.
The officer witnessed the shooting, got out of his vehicle, and intervened before he shot the suspect.
The school's PA system made an announcement that the school was on lockdown and th main entrance was closed by police. Parent Michael Evans said,
"The word kind of spread through the school that there was some kind of shooting outside"
This is not typically how you want meet the teacher to go.