Since Baltimore schools CEO Andrés Alonso took the helm four years ago, only about one-quarter of the system's principals have remained in their posts, a high turnover rate that has rankled education advocates who say they are concerned that leadership vacuums hamper progress.

With just 21/2 weeks until students return to the classroom, nine schools remain without permanent leadership assignments. Late Tuesday, Alonso appointed the 15th new principal in two weeks as the system races to fill a total of 42 vacancies that had opened this past school year alone — about half of those retirements or resignations.

"It's scary," said Marvin "Doc" Cheatham, former president of the Baltimore branch of the National Association for the Advancement of Colored People, who now heads the advocacy group National Action Network — Greater Baltimore Chapter. "Many of the principals are so experienced and so connected with the community that you never want to lose that."

Some principals who left the school system in recent months cited a range of reasons including constant pressure, lack of support and the exodus of colleagues in the past four years.

Alonso acknowledges the unprecedented turnover rate under his tenure compared with his predecessors but said the changes are necessary to improve schools and that it's imperative to have the right people in those positions. Alonso has increased the power of principals by giving them increased flexibility in their budgeting and staffing, arguing that the school leaders, as opposed to central office administrators, know how to best serve students.

Turnover "is still needed, especially given our outcomes this year and given how much work needs to be done," he said in an interview. "My job is to make tough decisions about leadership."

The principal changes come as the city looks to rebound from a backslide in academic progress and braces for reforms including a new national curriculum, radically different union contracts for teachers and administrators, and the pilot state evaluation system for teachers that will have student achievement account for 50 percent of their job rating. The city also is reeling from a series of cheating scandals in which test gains were inflated; that could lead to a few principals' losing their professional licenses.

The city experienced its first dip in years on the 2011 Maryland School Assessments, and a number of city schools individually saw sharp declines. Alonso has made holding principals accountable for achievement a trademark of his administration, but acknowledged that turnover in leadership and school staff contributed to some of the sharpest declines in school test scores this year.

Of the 188 school principals when Alonso arrived in 2007, 50 remain at the same schools they led four years ago. Another nine principals still lead schools in the system but have been transferred to new posts.

The vacancies included those created by reassignments, promotions and new schools being opened. The number of principals dismissed for poor performance has not yet been finalized, school officials said.

The turnover is unusual in the Baltimore metropolitan area. For example, eight of the 172 principals in Baltimore County are new for this coming school year, while in Anne Arundel County, 10 of 120 principals will be new to the job.

Education experts have said that high turnover rates in urban school systems are common when radical reforms are under way as they are in Baltimore.

"Turnover is always an unpleasant event to deal with," said Neil Duke, president of the city school board. "In any case, the bottom line is that the onus is on the district to make certain that we have effective leadership in each of our school buildings, and we will continue to support effective leadership across the district."

But Cheatham echoed a concern expressed by school employees and parents: The constant churning of principals is problematic given the unique challenges the city's student population faces.

"The people who head up our schools really have to have a love and compassion for the children and the neighborhood they're working in," said Cheatham, who recently called for the reinstatement of Montebello Junior Academy Principal Camille Bell, who was removed. Bell, who grappled this year with the fatal shooting of a 12-year-old student, has declined to comment, and school officials declined to discuss personnel issues, citing policy.

"If they're coming from the counties or even further, that means they have no understanding of the culture they're coming into," Cheatham said. "You can bring in some great PhDs and scholars, but if they're not connected to the community, it makes a difference."

The city's administrators union has attributed the turnover rate, in part, to Alonso's "impatience about results," which the CEO does not deny but says is necessary. Union officials have said that the aggressive style pushed a number of frustrated principals out of the system.

"For the past three years, I have consistently expressed my concerns over the astonishingly large number of seasoned administrators leaving the school system," said Jimmy Gittings, the union president.

"My No. 1 concern about the exiting of the seasoned administrators is that they are also taking their expertise and experience with them. Who now will mentor and guide our new and much younger administrators?" he said. He added that the union will give its full support to the new administrators as they transition into their jobs.

Under a new reorganization of the central office, the district has sought to provide more support as newer and less experienced principals are hired.