The board of directors of Noble Network of Charter Schools are reviewing why the founder and CEO of one of the oldest charter chains suddenly gave notice he’s “retiring for personal reasons.”

Board president Allan Muchin wrote in an email Tuesday to staffers that Michael Milkie will step down at the end of December after 20 years leading Chicago’s largest network of privately-managed charter high schools, which currently enrolls about 12,000 students. The email came after Tuesday morning’s regular monthly board meeting.

“Given the circumstances involved in his decision, we think this is best for Noble as well. The board intends to further review this situation,” wrote Muchin, who along with several other board members have Noble schools named for them.

A Noble spokesman could not immediately be reached for comment.

Noble’s president, Constance Jones, will take over as CEO, Muchin wrote, adding that “Noble remains strong as ever as evidenced by its outstanding results for students.”

Milkie and his wife, Tonya, were Chicago Public Schools teachers when they started the first of their now 18 schools, the Noble Street College Prep, in West Town, in 1999. Intended an alternative to traditional public schools, the idea was to use innovative techniques and cut through CPS’ bureaucracy to better educate kids. The rapidly growing chain gained a reputation for strict discipline, but also for getting low-income kids into college, especially undocumented students.

As faculty at some charter schools in Chicago have joined a union that’s now a subset of the powerful Chicago Teachers Union, Noble’s teachers have tried to organize but without success.