It was an emotional day in the trial of Abel Cedeno, the teenager charged with fatally stabbing a classmate inside the Urban Assembly School for Wildlife Conservation, a Bronx public high school.

"Honestly, it was, like, really heavy for me to sit there and watch them relive the day. They are not just classmates, they are best friends," said Louana Dennis, the victim’s mother.

Testifying for the prosecution, two classmates of Cedeno's described him as the aggressor in his fatal confrontation with 15-year-old Matthew McCree.

One of the classmates, Frankie Santiago, said the dispute began when a paper ball was thrown in the classroom near Cedeno.

The witness said Cedeno demanded, "Who threw it?" and that he was "very aggressive."

When McCree admitted that he did, Cedeno told him to "get up."

The witness said McCree walked toward Cedeno, who was holding a knife, and the two boys began fighting. But he said he did not see who threw the first punch.

An attorney for the victim's family called the testimony crucial.

"Abel Cedeno was the initial aggressor when he took out that and displayed it prior to being punched,” said Sanford Rubenstein, the attorney for the McCree estate. "Self-defense justification is not a defense that is available to him."

The second classmate, Aanaiya Santiago, paused at one point in her testimony to wipe away tears.

She said McCree apologized for throwing the ball, telling Cedeno, "Sorry, that was not meant for you."

But she said Cedeno used a vulgarity to call his classmates cowards and then shouted, "Pull up, pull up," which she described as a call to fight.

"To watch the expression on their face and to even hear, saw one of the witness crying, had me crying," said Louana Dennis.

The prosecution also played a video of Cedeno's confession.

Cedeno told detectives he bought the knife "to be safe, to have it on just in case."

"I was lost for words as I am sitting there watching the video, that he knew what he was doing. He had an agenda," said Dennis.

The defense says Cedeno had been relentlessly bullied by classmates because he is gay and feared for his life.

The trial will resume Tuesday with the testimony from Ariane LaBoy, the other student stabbed during the altercation.

Another classmate as well as a teacher is also expected to take the stand.