For Josephine Fanelli, the newly appointed principal of St. Theresa School, the toughest part of the job is likely to come on the sidelines of school basketball games.

At least when her team vies against crosstown rival Our Lady of the Assumption School, which counts among its players an 11-year-old named John Fanelli.

“I'll be rooting for St. Theresa's, but how can I not cheer for my son?” noted the principal, who is herself an alumna of OLA and who until last month was a teacher and dean there.

Fanelli has been getting her fair share of ribbing for her nine-block treason. Even her former boss, OLA pastor Msgr. Donald Dwyer, has joined in.

“I told her leaving OLA to go to St. Theresa was like Babe Ruth leaving the Yankees to go to the Red Sox,” he said, before dubbing her “a very capable educator” and admitting he's sad to see her go.

For good reason. Fanelli, who replaces Joe Mercora, arrives at St. Theresa School with over 20 years of experience in Catholic education, most of it teaching religion and language arts in grades six through eighth.

She is also certified as a Level II catechist with the New York Archdiocese, a distinction bestowed to those who study the Bible and Catholic belief extensively.

So far, despite her roots, Fanelli has received a warm welcome from parents, teachers and staff, as well as Fr. Robert Grippo, longtime pastor of St. Theresa Parish.

“Fr. Grippo is just a great person,” she said. “He's very helpful and he's very understanding.”

The lifelong educator said she sought the principal position because she loves her profession and felt she could make a greater impact by assisting a team of teachers.

“I've been teaching for a long time, and I just felt like it was time to move on,” she said. “And I felt like if I didn't do it now, I'd never do it.”

Geri Sciortino of Bronx Design Group, who is crafting a new faculty handbook for St. Theresa's, offered rave reviews of Fanelli's style, derived from her own children's experiences at OLA.

“My daughters said Mrs. Fanelli was the best teacher they ever had,” Sciortino said. “They said they wanted to come to this school because she's going to make it so fun.”

A self-described “Bronx girl,” Fanelli, who pegged her age in “the early 40s,” grew up in Pelham Bay and now lives in Country Club with her husband, John, and their two children, Stephanie, 13, and John.

She attended OLA, St. Catharine Academy and St. John's University. She also holds a master's degree in education from the College of New Rochelle and is pursuing a master's in school leadership at Mercy College.

St. Theresa has similar demographics to other local Catholic schools, including OLA, with an enrollment of about 350 students in grades pre-K through eight.

One concern parents have expressed is overcrowding in the upper grades, where teachers can find up to 45 students per class. Parents say the class size has led to a high teacher absence rate.

Fanelli said she hopes her experience teaching middle school-age children can help her address the challenge.

“I'm concerned with the numbers, but it's all about finances, and it's the situation we have,” she said. “My intent is to provide the teachers with as much help as I can, like help with grading papers so they can spend more time with the kids.”

Despite her lengthy career, or perhaps because of it, Fanelli discusses education with more enthusiasm than many veterans. She credits her faith and said her students' success has always propelled her to keep up her work.

“When that eighth-grader walks down the aisle at graduation, and they're successful — even the student who had the most problems — and I see their face, to me that's better than any paycheck,” she said.

Fanelli said she expects to remain at St. Theresa as long as she's welcome — comforting words to students and parents who will return in September to their third principal in as many years.

“As I say to the parents, I'm here to make St. Theresa the best school in the Archdiocese of New York,” Fanelli said. “Sorry, Fr. Dwyer.”