A teenager allegedly bedded by a horny female hoops coach at a Staten Island Catholic high school says he was double-crossed by District Attorney Daniel Donovan, who promised for months to throw the book at the educator but instead quashed the case two days after getting elected to Congress.

The minor and his mom met as recently as two weeks ago with Donovan, who reassured them he was seeking a grand jury indictment against Megan Mahoney, a vow assistant DA Wanda DeOliveira repeated the day before the election, they said.

But no jury was ever convened and on Tuesday, the day Donovan would be elected to Congress, the mother called DeOliveira and was told the case was being dismissed.

“I don’t even know what the point was in coming forward,” said the dejected student, now 18. The Post is withholding his name.

Mahoney, 25, was arrested on Oct. 20 following a Post exposé about Moore Catholic High School, where the former teacher and basketball coach allegedly had sex with the then-16-year-old boy about twice a week for months, sometimes on school grounds. She previously was accused of seducing another underage boy.

The youth and his mother arranged a 45-minute sit-down with Donovan and DeOliviera on April 27 after becoming concerned that Mahoney would be offered a free pass known as an adjournment in contemplation of dismissal.

During the meeting, the teen said, he repeatedly offered to testify before a grand jury and provided names of witnesses. He even produced a love note on a Christmas card Mahoney allegedly sent him.

But the witnesses were never interviewed, he said, including his ex-girlfriend, who had been following him and Mahoney and called the cops.

Donovan and DeOliviera shrugged off the love note and complained that the youth at first denied the romps.

“Donovan said, ‘I would have given the same answer — I wouldn’t have admitted it. But in legal terms, it puts us in a bad light,’ ” the mother recalled. “But my son said, ‘Trust me. Let me go to the grand jury. They will believe me because I’m telling the truth.’ ”

A source who knows Mahoney’s father, retired NYPD Sgt. John Mahoney, said the ex-cop is close with Donovan from his days in law enforcement.

“Everybody knew he was going to reach out: What can you do for my daughter?”

The mother said John Mahoney has “got a lot of connections, a lot of power.”

The Mahoney family’s lawyer denied that the father intervened. “Absolutely not,” said Eric Nelson. Megan Mahoney refused to comment.

“The decision to offer a [dismissal] in this case was reached after a full investigation … there is no DNA evidence, no admissions by the defendant, no eyewitnesses to the alleged acts, no forensic evidence, and no recordings,” said a Donovan spokesman. “It should also be noted that there was no immediate outcry by the complainant, who waited nine months before reporting the incident to the police.” The dismissal and sealing of the case “is in the best interest of justice,” he added.