(CNN) 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.

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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.