BARABOO, Wis. — As he looked down the barrel of the handgun Eric Hainstock was pointing at him, Weston High School Principal John Klang stood in the school hallway with his hands up and said, "You don't want to do this," a student testified Monday.

Joshua Manock said that minutes later, he heard three separate gunshots as he ran with other students for cover on Sept. 29, 2006. (VIDEO)

Prosecutors allege Hainstock, 16, knowingly fired those shots with a .22-caliber handgun and killed Klang, 49, that day in the small farming community of Cazenovia, Wisconsin.

Hainstock has pleaded not guilty to intentional first-degree homicide.

His attorneys contend that, although the teen's decision to bring a loaded rifle and handgun to school was reckless, he did not intend to shoot and kill his principal. They claim the gun fired accidentally when Klang attempted to wrestle it away.

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Manock testified that, when he came upon Hainstock in the school hallway that morning, Hainstock swung around and pointed a handgun at him.

As Manock turned away and sped down the hallway, Klang rounded the corner and asked the teenager to slow down. But as he raced past his principal, Manock said, he could only manage to utter the word "gun."

Manock was able to hear one further exchange between Hainstock and Klang as he fled the school with other students and teachers.

Klang asked the armed teenager, "Is the gun real?", to which Hainstock shouted back, "What the f--- do you think?"

Shortly afterward, Hainstock's special education teacher, James Nowak, arrived at the scene.

Nowak testified Monday that Hainstock pointed the gun in his direction. Nowak "never said a word," turned and started running toward his classroom, he said.

The last thing he saw before hearing two gunshots was Klang "take two steps and then jump" toward Hainstock.

Hainstock's attorneys argue that Hainstock was a troubled teenager who suffered from constant teasing and bullying at school, abuse at home and a severe case of attention deficit hyperactivity disorder.

In their opening statements, they argued that Hainstock didn't come to school with guns to shoot anyone, but in hopes someone would take his problems seriously.

Manock testified that, although he didn't think Hainstock was the victim of regular teasing, he did recall a day in 2006 when he saw students picking on Hainstock and Manock told them "to leave him alone."

But under cross-examination, it became clear that Manock was aware Hainstock was a common target of teasing. In an interview Manock gave police after the shooting, he said students regularly picked on Hainstock, who reacted in a self-deprecating manner.

"If a person said Hainstock would stink, Hainstock would agree with them," Manock told investigators.

Another student, Samuel Brandt, acknowledged that Hainstock was teased, but said Hainstock also instigated teasing. Brandt added that it was common for everyone in the school to be teased at one time or another.

Kimberly Durst, who was in the same gym class with Hainstock, supported this assessment, saying that, although Hainstock was "picked on," he "picked on others."

Durst said she never took Hainstock's threats seriously until the Friday before the shooting, when Hainstock spoke of harming his special education teacher, Nowak.

She said Hainstock was upset with Nowak because he believed the teacher was responsible for his receiving an in-school suspension for being caught with chewing tobacco. Hainstock had thrown a metal stapler at Nowak the week before the shooting.

Durst said she told Hainstock, "Eric, stop messing around. We don't need another Columbine here."

When Hainstock said he didn't know what Columbine was, Durst said she explained that two teens had gone on a shooting rampage at their Colorado high school.

Hainstock replied soberly, "Those people were stupid for doing that," she said.

Jane Huth, Weston School District secretary, testified that she had given Hainstock bandages for what appeared to be self-inflicted wounds on his arms.

The trial is being streamed live on Court TV Extra.