After 18 sometimes-rocky months, the Philadelphia School District's top charter school administrator has resigned, according to district and charter sources.

Benjamin W. Rayer's departure comes amid reports that Superintendent Arlene Ackerman is readying yet another round of administrative shake-ups.

District spokesman Fernando Gallard declined to comment.

"We are not ready to comment on senior staff changes at this point," Gallard said in an e-mail, adding that the district plans to make an announcement "within the next several days."

Rayer, 43, a former chief operating officer at Mastery Charter Schools, did not respond to e-mails or phone calls seeking comment.

After he was appointed, Rayer said his goal was to improve charters' relationships with the district, which were often strained.

During his tenure, he strengthened communications between the district and charter operators and made it a point to visit their schools.

Rayer successfully pushed the School Reform Commission to adopt a new charter policy spelling out rules that charters must follow for renewals and expansions, although the policy rankled some school operators.

After Rayer in June told operators in a conference call that the commission would not be able to vote on charter expansions, Ackerman publicly disputed the account.

The commission voted as scheduled. And Ackerman said, "The SRC never, ever directed the staff that this vote was not going to take place on June 16."

Rayer presided during a tumultuous time for charters in the district. A federal investigation of some charters that began before he arrived expanded from three to 17 schools.

In April, City Controller Alan Butkovitz blasted the district's oversight of its 67 charter schools. He said the district's failure to monitor those schools left both the district and the taxpayers "extremely vulnerable to fraud, waste, and abuse."

Rayer also was responsible for leading the Renaissance schools initiative, which culminated with the announcement in the spring that 14 low-performing schools would undergo dramatic makeovers, including West Philadelphia High School.

Ackerman's team was embarrassed when the overhaul of West had to be delayed after reports surfaced that some members of a parents' advisory council who were supposed to decide who should run West had conflicts of interest because they received stipends from one of the proposed providers.

In late June, Ackerman announced that Diane Castelbuono, a deputy secretary in the Pennsylvania Department of Education, was joining the district as associate superintendent of strategic programs. In that role, Castelbuono, who once ran the district's charter office, will oversee charters as well as Renaissance and turnaround schools.