A judge entered a plea of not guilty on Nikolas Cruz’s behalf as the teen was arraigned Wednesday in last month’s massacre at a South Florida high school — on the same day students across the country walked out of class to demand stricter gun laws because of the killings.

A Broward County grand jury last week indicted the 19-year-old gunman on 17 counts of premeditated murder in the first degree and 17 counts of attempted murder in the first degree.

Wednesday’s plea in a Fort Lauderdale court came after Cruz’s defense attorney told the judge that the teen was standing mute to the charges — meaning he was declining to enter a plea.

Cruz, wearing a red jail jumpsuit, largely kept his head down as he sat in a jury box with three law enforcement officers standing behind him.

Some relatives of the victims also sat in the courtroom, watching the proceedings. A few of them wiped tears after Cruz entered the room.

Also watching was Cruz’s younger brother, Zachary Cruz.

Nikolas Cruz killed 17 people — both students and school staff — at Marjory Stoneman Douglas High School in Parkland on February 14, in one of the deadliest mass shootings in modern US history.

He confessed to being the gunman, according to a probable cause affidavit released shortly after his arrest.

Prosecutors to seek death penalty

Prosecutors announced in court filings Tuesday that they will seek the death penalty. They listed several factors, including that Cruz knowingly created a risk of death for many people and that the shooting was “especially heinous, atrocious or cruel.”

Read the notice of intent for yourself

The head of the county’s public defender’s office, which is representing Cruz, said last month that there is no question Cruz is guilty, and that he was willing to plead guilty to avoid the death penalty.

From ‘broken child’ to mass killer

Cruz is being held without bond at the Broward County Jail, where he’s segregated from other inmates.

The teen was arrested shortly after committing the Valentine’s Day killings and fleeing the high school campus among terrified students.