A preschool teacher at elite Stanford University was arrested for murder — while her kiddie students were off napping, officials say.

Diana Ornelas, 22, was nabbed in the killing 24-year-old Oliver Waterfall as she worked Wednesday for the daycare center at the top California college, according to The Stanford Daily.

“The arrest happened at naptime, and outside of the perimeter of the CCSC [Children’s Center of the Stanford Community] campus. Children were not present when this occurred, and it did not disrupt operations,” CCSC officials wrote to parents.

The childcare facility caters exclusively to the children of Stanford staff and students as an independent non-profit, its website says.

Waterfall’s body had been found along a Santa Cruz County highway April 11. He had been shot dead about a month before, officials believe.

Police suspect that Ornelas was friends with both the victim and Dennis Novoa, 27 — who was also charged in connection with Waterfall’s shooting death, the outlet said. The motive was not immediately clear.

Ornelas was fired from her job after the arrest, the letter to parents said.

She was ordered held without bail.

She and Novoa are set to be arraigned May 17.