The educator -- who worked at the swanky Saint Ann’s School in Brooklyn Heights -- turned himself over to Brooklyn prosecutors on Thursday ... a month after he was accused of catfishing minors and sharing X-rated images with them through social media.

Now, he is facing child porn charges and is expected to be arraigned in Brooklyn Criminal Court on Thursday afternoon .... where he'll be charged with using a child in a sexual performance and for promoting a sexual performance by a child.

Per The New York Times, Nguyen has been accused of soliciting X-rated images from 5 girls and 1 boy, who were students at 4 different schools. He allegedly pretended to be a teen online ... trying at least 11 times to get students to send him explicit images.

Nguyen was first arrested in front of his students at Saint Ann’s ... but he was later released as authorities began a probe into his personal devices. That being said, he was put on administrative leave at Saint Ann's as the investigation continued.

"Jeopardy!" fans likely recognize Nguyen from his 2 appearances on the quiz show, which took place in 2014.

This isn't Nguyen's first run-in with the law either ... as he was previously booked in 2017 for stealing over $300K from an elderly couple -- for which he did a stint at Rikers Island.