NY- A sex scandal has exploded at a prestigious upstate Catholic school, where randy teacher Sandra “Beth” Geisel bedded four students – including an underage boy – while administrators tried to keep the seamy details under wraps, authorities say.

Geisel, 42, a stunning blond married to a powerful Albany banker, was booted from the Christian Brothers Academy in Colonie in June – immediately after a backseat sex romp with a 17-year-old student became public.

Geisel, who was on the payroll first as an English teacher and then as a volunteer in the school’s writing lab, now faces criminal charges involving a 16-year-old boy. She’s expected to surrender today on felony rape charges, according to a lawyer for the family of another student.

John Aretakis, who has represented alleged victims of clerical sexual abuse and is now the lawyer for a 17-year-old CBA student – allegedly one of four students who had sex with the vivacious mother of four – says her “bizarre and irresponsible behavior” was ignored for months by the school.

Geisel, an immensely popular English teacher before she was axed, got canned after police in Cohoes – a town adjacent to Colonie – responded to a resident’s call June 11 about a suspicious parked car.

A cop found a sex-rumpled Geisel and a 17-year-old student inside.

No charges arose from the case because the boy was at the age of consent for sexual activity. Geisel reportedly confessed to school officials and was immediately fired.

About a week after the tryst, CBA sent a letter to parents about the affair, though officials didn’t identify the teacher. Officials said they’d found the conduct inappropriate.

However, after reports of Geisel’s dismissal spread, a 16-year-old student, at the urging of his parents, reported to Colonie police that he’d had two romps of his own with the randy teacher, according to yesterday’s Troy Record.

A spokesman for the Colonie police said that he believed Geisel was expected to surrender today but that he had no details. Charges reportedly could include statutory rape involving a minor. Geisel’s lawyer, Donald Kinsella, did not return repeated calls for comment. He told The Troy Record she would voluntarily surrender.

Aretakis alleges the teacher’s out-of-control sexual behavior was known among students and their parents for months – and had to be known by administrators – yet was kept secret.

“I believe it is similar to what’s going on . . . in the Catholic church, that this Catholic school was looking to avoid shame and embarrassment and that they tried to wish these incidents away,” Aretakis said.

“They said, ‘Let’s keep this in the family. Use restraint. We believe this incident is isolated.’ I’m angry for them acting irresponsibly.”

According to Aretakis, the inappropriate behavior allegedly began in February on a school-sponsored trip to Florida, where Geisel allegedly drank and, clad in a bikini, posed for a picture with her hand on one of the students.

“She’s in a position of trust and power, working in a Catholic school – and it is so reckless and irresponsible,” said Aretakis, adding that four families are now considering filing a civil lawsuit.

On the online RateMyTeachers.com, Geisel won lascivious praise from one – and real admiration from others.

For example, one student went on line in March to pant, “down boy down – get what I’m saying guys.” Another, last November, declared her “much more” than “just a pretty face.” Another, in February, called her an “amazing writer.”

Geisel is reportedly estranged from her husband, Thomas Geisel, 43, who heads commercial banking in the Northeast and New England for KeyBank and is president of KeyBank in the Albany region.