A Pierce County judge dismissed four assault charges against a 16-year-old accused of stabbing four students at Tacoma’s Foss High School, after prosecutors said newly reviewed Instagram messages undermined their ability to prove the case. Prosecutors asked Judge Michael Schwartz to dismiss the charges Friday, saying the messages indicated the students who were injured had been planning to rob the teenager days before the April stabbing. In a motion signed by Pierce County Prosecuting Attorney Mary Robnett, the office said it could not prove beyond a reasonable doubt that the teenager was the initial aggressor or rule out self-defense. The teen was being prosecuted as an adult and had faced four counts of first-degree assault, one for each injured student. A school security officer was also cut while breaking up the fight. The officer said he did not believe the teen intended to stab him. Surveillance video showed the four students surrounding the 16-year-old in a school hallway, according to earlier court filings. The teen and one student lunged at the same time, and the student was stabbed in the chest. The injuries to the other students included multiple stab wounds, a deep forearm cut requiring surgery and a back wound. The student stabbed in the chest had part of a lung removed. The teen’s lawyer had argued that the students planned an attack over a dispute involving a vape battery. A June motion cited a witness who told police the students were trying to set the teen up, as well as a security officer’s account that the teen was crouched and protecting his head during the fight. Those claims were contested allegations; prosecutors’ decision to dismiss followed further police investigation of the messages. The teenager was initially held on $750,000 bail. In June, Judge Jennifer Andrews reduced bail to $250 after the defense argued that charging documents had left out evidence supporting a self-defense claim. The teen had been on home detention before the charges were dismissed.