Dombrowa said the officer did not know there was a drill taking place.
"He's a police officer," she told the Chronicle. "He did what he was supposed to do."
The officer heard through the school's intercom system that the campus was being placed on lockdown, KPRC reported.
Dombrowa said the hostage-taking student was carrying a blue wooden handgun, but the officer couldn't see the fake gun at the time, she said.
"You have to commend the officer," Dombrowa told KTRK. "He responded to what he thought was an active scene and realized almost immediately it was a drill."
The two high school students acted out their parts in the school's hallway as elementary school students locked themselves in their rooms, Dombrowa told the Chronicle.
Dombrowa told the newspaper that the district got permission from the student actors' parents before the drill and posted notices of the simulation on its website and sent letters home to parents.
"I suspect there will be some changes that come out of this," she said, later adding, "We learn something every time we do a drill, and that's the purpose for doing them."