IT WAS JUST 17 months ago that Jeremy Nowak strolled into the once-staid offices of Philadelphia's biggest locally oriented philanthropy, the William Penn Foundation, as its new president - a big man with big, radical ideas for change.

In a short time, the former community-development guru thrust the $2 billion foundation into the center of the fight over school reform in Philadelphia - gaining both powerful allies and a few harsh critics, and putting the William Penn Foundation in the headlines.

And now, abruptly, he's gone.

In a move that stunned Philadelphia's corridors of clout, the foundation announced Wednesday that Nowak was out as president - or, in "philanthropese," they had "mutually decided that the time is right for Nowak to transition out of his current role."

Its release cited "differences in approach" in carrying out the plan that was developed under Nowak for the foundation - which doles out about $80 million a year to local causes - to focus on school reform, preserving the Philadelphia-area watershed and backing neighborhood-level cultural groups.

Unofficially, the breakup was blamed on a culture clash between the forceful, headline-grabbing Nowak and the lower-key board. The foundation was founded in 1945 by Otto and Phoebe Haas, of the Rohm & Haas chemical fortune, and is still closely tied to their heirs, including current chairman David Haas.

"It's about style, not substance," said Brent Thompson, the foundation's director of communications. He insisted that there would be no change in policies and direction of the William Penn Foundation - a matter of intense interest to parents and teachers who've seen the philanthropy take a front-and-center role in mapping out the future of Philadelphia's financially reeling school district.

Under Nowak, the foundation raised more than $1 million to pay the Boston Consulting Group, which devised a plan for the Philadelphia School Reform Commission that called for closing as many as 64 schools and placing more kids in privately managed classrooms. It also funneled $15 million to the Philadelphia School Partnership, which is largely supportive of charters and nonpublic schools, while groups that back traditional public schools fear that their grants will vanish.

Helen Gym, the co-founder of Parents United for Public Education, said that "William Penn, under [Nowak's] stewardship, went from being this beloved Philadelphia foundation to being a controversial and very conservative promoter of a very special kind of reform agenda." Her group recently teamed with the Public Interest Law Center of Philadelphia to accuse the foundation and Boston Consulting of improperly lobbying on school issues.

Thompson said that a search is under way for Nowak's successor but that there's no immediate timetable. In the interim, longtime foundation employee Helen Davis Picher, most recently director of evaluation and planning, will act as president.

Even as officials worked to downplay its importance, the sudden move sent shockwaves through the world of nonprofits, many of which receive funding from or are closely tied to the William Penn Foundation. Said Helen Cunningham, the executive director of the philanthropic Samuel S. Fels Fund: "A grant from them is an imprimatur of sorts. It sends a message to national foundations that this is an organization of high quality."