In the summer of 2012, Phoenix Xavier Prep long-time athletic director and golf coach Sister Lynn Winsor received a call from her school’s admissions office about an incoming freshman from Cupertino, Calif., in the San Francisco Bay area.

“They said that she had taken her entrance test and that she plays golf,’’ Winsor said. “I said, “Well, all right. OK. I guess we will find out what she can do.’ ’’

It so happened that Hannah O’Sullivan could play the game a little. OK, more than a little. Winsor did some research and discovered that O’Sullivan was one of the fastest rising stars in the nation.

And now she was at Xavier, which had no shortage of talent over the years with 29 state championships at the time.

Winsor called O’Sullivan’s arrival “the surprise of the century.’’

It was no surprise, then, that by season’s end, Xavier had earned its 30^th Division I title and O’Sullivan was the individual champion. She shot a first-round 68 at the Aguila Golf Course in Laveen and then took everyone’s breath away with a final-round 64.

There is a small hill near the green at No. 18 and there were a few dozen supporters waiting for the last putt to drop.

Once it went in, they ran down the hill to congratulate her.

It probably was the only time she was nervous, seeing all of them at once.

“I was kind of scared,’’ O’Sullivan said. “I sort of thought they were going to knock me over.’’

Tui Selvaratnam, Winsor’s co-coach, said she was amazed by what O’Sullivan did during the season, but even more amazed by what she did at the state tournament.

“Especially that final day,’’ Selvaratnam said. “I knew she would play well, but not that well. She just kept that focus.’’

It was another noteworthy achievement in a long list of noteworthy achievements and what is sure to be many more. O’Sullivan played in the U.S. Women’s Open at Blackwolf Run in Kohler, Wis., in the summer of 2012 and shot 76 and 78. In the summer of 2013, she played in the USGA Women’s State Team Championship in Kettering, Ohio, and the PING Invitational in Oklahoma.

Now a 15-year-old sophomore, O’Sullivan is confident, yet humble and takes nothing for granted.

“I love golf so much. It has already given me so many wonderful opportunities,’’ she said.

She tries not to lose her composure, even when things do not go as smoothly as she would like.

“As bad as it could be, I think that it is nothing compared to all the good days that I have had or will have,’’ she said. “I enjoy seeing my hard work pay off.’’

She also works hard in the classroom. She carried a 4.33 grade-point average last year and is approaching that again this year.

Playing at Xavier “is a lot of fun. I have made some great friends, and it is a great team atmosphere.’’

As good as O’Sullivan is, Winsor said she is improving by playing against quality teammates in practice. Junior Ariana Macioce finished fifth last season and junior Madison Kerley was seventh.

O’Sullivan started playing mini-golf at the age of 6. She also was involved in soccer, figure skating, gymnastics, basketball and softball.

“But I fell in love with golf, and I decided to stick with it,’’ she said.

When the wins started to come around the age of 8, it sealed the deal.

O’Sullivan’s father is a software engineer and his job brought him to Arizona. He researched the good golf schools, through Google, and Xavier kept coming up.

The state tournament returns to Aguila on Oct. 30 and O’Sullivan is looking forward to helping the team decorate the bus the day before. The players also have a tradition of making cookies and distributing them to golf-course workers and others along the way. O’Sullivan’s favorites are chocolate chip and snickerdoodles.

She already is getting plenty of attention from colleges. She wants to go to a good golf school, but says, “that is only part of the decision. I want it to be a good school for academics, and I want to enjoy the area where the school is. After all, I will be spending four years of my life there.’’

And, of course, playing on the LPGA Tour is the ultimate goal. She has followed the career of current LPGA player Stacy Lewis and is impressed by how intelligent she is and how she gives back to the community.

“I want to be like that,’’ O’Sullivan said.

Winsor has no doubt that it will happen.

“She is one of the premier golfers we have had and has a chance to be the best,’’ said Winsor, who has coached Heather and Missy Farr, Amanda Blumenhurst and Cheyenne Woods.

“Her work ethic is tremendous. She has that drive. She likes to have fun, but out on the course, she is all business. I can easily see her in the LPGA.’’

Winsor said many of O’Sullivan’s opponents realize it, too. Some have asked her to sign their scorecards.

“Because they know she is going to be famous,’’ Winsor said. “So that some day they can say, “I played against her in high school.’ ’’