Randi Coy (search), the star of "My Big Fat Obnoxious Fiancé," (search) was forced to resign yesterday from her teaching job at a Catholic school in Phoenix, Ariz.

Coy, a first-year, first-grade teacher at Pope John XXIII Catholic School, took a leave of absence over Thanksgiving to tape "Fiancé," which premiered Monday night on Fox.

"She was given a deadline of whether or not to come back," Archdiocese of Phoenix spokeswoman Mary Jo West told The Post yesterday.

"She officially called the principal of her school . . . and told him it was a tough decision - but that she decided it was best for her to resign her position," West said. "They apparently had promotional appearances [scheduled] for her and needed to keep her."

West said Coy, 23 - an Arizona State University (search) graduate - was hired by the school in August.

The school was not too thrilled that she was participating in the reality show - on which the attractive young teacher is trying to con her parents into thinking she wants to marry an inappropriate guy.

The school's principal, Bill Langley, sent a letter to parents of her students telling them about Coy's leave and her appearance on "Fat Fiancé," according to The Arizona Republic.

"You can't be teaching and do a show like that," Langley told the paper.

"We brought in two different substitute teachers, then at Christmas we brought in a permanent teacher to take over if [Coy] decided not to come back," the spokeswoman said

On the show, which drew 19.6 million viewers in its debut, Coy stands to win $500,000.

Fox will re-air Monday's premiere tonight at 9 on Ch. 5.