On a winter night three years ago, a 17-year-old English High School student stumbled into a Roxbury street and flagged down an oncoming SUV as blood gushed from a gunshot wound to his head.

“I don’t want to die. Please help me. I don’t want to die,’’ the teenager said, according to Jeffrey Bullock, the 28-year-old landscaper who was waved over by the injured student.

Bullock described his dramatic encounter with the teenager Thursday as testimony began in the trial of Shaun O. Harrison, 58, the former English High dean accused of shooting the student execution-style on March 3, 2015.

Prosecutors allege that Harrison, a self-styled preacher and antiviolence activist, recruited the student, Luis Rodriguez, to sell drugs for him at English High in Jamaica Plain, then shot him in a dispute over slumping sales.

Harrison has pleaded not guilty to charges of armed assault and illegal possession of firearms, ammunition, and drugs.

During opening statements in Suffolk Superior Court, defense lawyer Bruce W. Carroll said the prosecution’s case has holes in it as a jury of nine women and five men listened.

Rodriguez, who has told investigators that Harrison shot him, is expected to testify, according to Suffolk Assistant District Attorney David Bradley.

He told jurors that Harrison began working at English High in January 2015 as the dean of academies, and was responsible for helping troubled students like Rodriguez, whose mother was in jail.

But instead of shielding Rodriguez from harm, Harrison enlisted him to sell drugs after he had been dean just a month, Bradley said.

“This case involves Mr. Harrison taking advantage of a young and troubled kid. Having him sell marijuana for him [and] getting in a disagreement with Mr. Rodriguez to the point where he walks him to a cold and dark place, looks behind him, and shoots him in the back of the head,’’ Bradley said.

In court papers, prosecutors have said that Harrison was a member of the Latin Kings gang, but before the trial, Superior Court Judge Christopher J. Muse ruled that jurors can’t hear evidence linking him to the group.

On the day of the shooting, prosecutors allege that a student attacked Rodriguez — at Harrison’s direction — while school was in session. After class, Harrison and Rodriguez made plans to meet to get drugs and socialize with women, Bradley said.

Surveillance video captured the shooting on Magazine Street about 7 p.m., he said.

“Mr. Rodriguez collapses instantaneously. The defendant leaves him suffering with a bullet in his cheek,’’ Bradley said. “The defendant runs. He runs from the scene leaving Mr. Rodriguez to die in the street.’’

Rodriguez flagged down Bullock, who was driving to Olive Garden with his girlfriend, Sharese Strickland, according to court testimony.

Strickland testified she called 911 and photographed the victim, who was bleeding from his ear. Rescuers brought him to Boston Medical Center, where he told his aunt that “Rev’’ shot him, Bradley said.

After the shooting, investigators interviewed Harrison and seized guns, ammunition, and drugs while searching his Roxbury apartment and basement storage locker, he said.

Police also arrested three men seen leaving Harrison’s apartment, though charges against them were later dropped after a judge ruled that they were illegally stopped and frisked by a detective.

The weapon used in the shooting has not been recovered, prosecutors have said.

Ballistics testing has shown that neither of the guns allegedly found in a safe at Harrison’s home matched the gun used to shoot Rodriguez. However, ammunition of the same brand and type as a shell casing discovered at the scene was in the safe, prosecutors said.

Carroll said the evidence against Harrison isn’t credible.

“You’re going to find that what Mr. Bradley has so forcefully put in front of you is not going to hold up,’’ he said.

On cross-examination, Carroll highlighted Rodriguez’s ability to speak with police, bystanders, and rescuers who came to his aid. In some of those conversations, Rodriguez said he didn’t know who shot him, several witnesses testified.

Bullock testified that Rodriguez said he didn’t remember whether he was with anyone before he was shot. Under questioning from Carroll, a paramedic who treated Rodriguez testified that his partner wrote in a report that the victim was “shot by an unknown person.’’

Testimony is scheduled to resume Friday.

Laura Crimaldi can be reached at laura.crimaldi@globe.com. Follow her on Twitter @lauracrimaldi.