Brenden Bridges and Adam Staup pleaded guilty and face up to life in prison for killing 17-year-old Ivan Mejia.
The then-16-year-olds lured Mejia behind Wylie East High School and tried to bury his body near Firewheel Town Center in March 2014.
Police said the killers planned the murder because Bridges liked Mejia's on-again, off-again girlfriend.
The girl, who is now 16, took the stand on Tuesday, saying she never knew about the murder plot.
Testimony resumes Wednesday morning.