The prosecution's star witness took the stand Tuesday in the murder trial of Abel Cedeno, a teen accused of a fatal high school stabbing in 2017.

Cedeno is accused of fatally stabbing 15-year-old Matthew McRee and injuring another student, Ariane Laboy, during a fight inside a classroom at the Urban Assembly School for Wildlife Conservation in September 2017.

Laboy took the stand Tuesday to testify about the classroom fight that killed his friend and left him severely injured. Laboy, who is now 18, broke down on the stand several times describing the stabbing of McRee.

The prosecution kept the media away from Laboy by bringing him through an entrance, avoiding television cameras.

Laboy told the court how he saw McRee holding his chest after he was stabbed. He says he got up and began to fight with Abel Cedeno. 

Laboy showed the judge scars from several stab wounds, which he says were inflicted by Cedeno.

During cross-examination, Laboy was grilled by Cedeno's lawyers for close to two hours. The defense claims Laboy and McRee were the aggressors and systemically bullied Cedeno at the school because he was gay. The prosecution complained several times to the presiding judge that Cedeno's lawyer, Robert Feldman, was harassing Laboy on the stand by his tone of voice and a series of what they deemed irrelevant questionings.

A substitute teacher also took the stand Tuesday and testified that Matthew McRee threw the first punch at Cedeno.

Both opposing sides have a civil lawsuit against the Department of Education, claiming it failed to protect students.

On Wednesday, several more students who were in the classroom when the stabbing took place are scheduled to testify.

MORE: Classmates testify in deadly school stabbing trial, Trial begins for Bronx teen accused of fatally stabbing classmate , Cedeno to take stand in his own defense in fatal school stabbing trial , Judge considers confession video evidence in 2017 fatal school stabbing , Special prosecutor request denied in case of fatal stabbing in Bronx classroom