A second-grade schoolteacher sobbed on the witness stand yesterday as she recalled a harrowing sex attack at the hands of a drunken off-duty cop who threatened that he’d shoot her in the face.

“He said, ‘You’d better come with me,’ ” the 25-year-old woman told jurors of the first moments of the attack, in which lecherous lawman Michael Pena approached her on an Inwood street as she headed to her first day of work at a Bronx charter school one early morning in August.

“Then he lifted his shirt and showed me his gun,” cried the woman, a petite, bespectacled brunette.

She broke into tears repeatedly during her testimony, her face contorting, her soft gasps amplified over the witness-stand microphone.

She said that as the off-duty officer dragged her at gunpoint into an alleyway behind an apartment building, she tried to give him anything she could to make him go away.

“I tried to offer my cellphone. My money. My wallet,” she told the jurors in Manhattan Supreme Court.

She also remembered turning toward a distant passer-by and starting to scream — but he quickly scared her into silence.

“He pulled out the gun and said, ‘Shut the f–k up or I’ll shoot you.’ ”

She told jurors that she begged him to stop, pleading with him, “Please, no! No! No!”

“Then after he told me he would shoot me in the f–king face, I didn’t say anything,” she testified.

After residents at the apartment building called 911, cops caught Pena at the scene zipping his pants up, witnesses say.

He was implicated by the DNA he left on the woman’s clothing, and after claiming that the sex was consensual, he admitted he dragged her to a back alley and attacked her.

But he’s trying to outrun the top charges of predatory sexual assault and rape — which carry anywhere from 10 years to life in prison — by claiming that he never actually managed to have intercourse with her.

“Were your knees buckling? Were you trembling? Were you scared?” defense lawyer Ephraim Savitt asked the woman repeatedly on cross-examination, apparently trying to show her as too distraught during the attack to remember if she had been physically violated.

“Petrified,” she answered. “When I realized he had a gun, yes.”

But prosecutor Evan Krutoy made sure the jury heard her say again, and with certainty, that the statutory requirements for rape and predatory sex assault had been met.

“Is there any doubt in your mind?” Krutoy asked her, going through each of four violations charged against the drunken cop.

“No,” she answered, her voice firm.

“You felt that?” the prosecutor asked again.

“Yes,” she answered.

“He told me to shut my eyes or he’d f–king shoot me,” the victim testified — and so she kept her eyes shut throughout the attack.

When the prosecutor showed a surveillance video of a young woman with a long braid walking down a shady sidewalk, followed by a man in a purple shirt and with a shaven head, the victim identified the figures as herself and her attacker.

Asked if she saw her attacker in the courtroom, she looked Pena straight in the eye, pointed at him, and said, “Yes.” Then her face collapsed again into a silent sob, and she balled up both her fists and held them to her face, her head bowed.

Pena, 28 — a three-year cop who worked out of the 33rd Precinct — remains jailed in lieu of $1 million bail bond. He has also been suspended without pay.