VAN NUYS >> Former Miami Dolphins player and Harvard-Westlake School alum Jonathan Martin pleaded not guilty Tuesday to criminal threats charges stemming from an Instagram post that led to a daylong shut down of the exclusive Studio City campus.

Jonathan Martin, 28, is facing four felony counts of criminal threats and a misdemeanor count of possessing a loaded firearm. He is due back in court in Van Nuys April 25, when a date will be set for a hearing to determine if there is enough evidence for him to stand trial.

Martin faces up to six years behind bars if convicted as charged, according to the District Attorney’s Office.

On Feb. 23, officials with Harvard-Westlake sent an early morning message via the school’s emergency alert notification system advising students to stay home, apologizing for the short notice and citing student safety as the school’s highest priority.

According to online images of the Instagram message, Martin posted a photo of a shotgun with a several shotgun shells, emblazoned with the hashtags #HarvardWestlake and #MiamiDolphins. The text of the post read, “When you’re a bully victim & a coward, your options are suicide, or revenge.”

Martin, who retired in 2015, was at the center of an NFL bullying scandal during his playing days and claimed during that investigation that he had also been bullied at Harvard-Westlake.

The Instagram post tagged former Dolphins teammates Richie Incognito and Mike Pouncey and a pair of former Harvard-Westlake classmates. Incognito was suspended by the Dolphins in 2013 in response to allegations of bullying and harassment — some of it alleged to be racial in nature — targeting Martin. Pouncey was also implicated in the scandal that led to extensive investigations by the NFL and the Dolphins.

Incognito did not play the entire 2014 season, but later signed with the Buffalo Bills, where he still plays.

Law enforcement sources told the Los Angeles Times that Martin was detained and briefly hospitalized at a mental health facility after the Instagram post came to light, and he was found with a loaded weapon in a car in Glendale. Detectives searched his home and found other weapons, an ax and a large knife, The Times reported.