The ex-fiancee of a prep-school teacher who is accused of bedding a 16-year-old student landed on the hot seat herself in Manhattan court Wednesday for lying under oath twice.

The Manhattan District Attorney’s Office already brought perjury charges against Marina Bontkowski for lying to a grand jury to protect her then-beau, Richard Hovan, the pretty brunette admitted on the stand.

“So really, the oath you’ve taken … now really means nothing to you,’’ said defense lawyer Vinoo Varghese.

“That’s not true,” Bontkowski replied defensively. “I pled guilty because I couldn’t look at myself in the mirror knowing that I had lied.”

Hovan, 33, a former Riverdale Country School teacher, is on trial for allegedly having sex with his student.