Noble Network board Chairman Allan Muchin announced today that longtime CEO Michael Milkie will retire at the end of the calendar year.

The letter said the decision was “for personal reasons” but didn’t describe the exact circumstances behind Milkie’s sudden departure, and Noble leaders weren’t reachable for comment Tuesday. 

“Given the circumstances involved in his decision, we think this is best for Noble as well,” Muchin wrote. “The board intends to further review the situation.” 

Noble Network is the district’s largest charter operator, with 12,000 students, 17 high schools, and one middle school.

Milkie and his wife, Tonya, were former Chicago Public Schools teachers who opened their first charter campus in 1999 in the city’s West Town neighborhood. They began expanding in 2006 and were buoyed by the Renaissance 2010 plan spearheaded by Mayor Richard M. Daley and his schools chief Arne Duncan that seeded many new charters. 

Noble President Constance Jones, a more recent hire, will step into the CEO role. Ellen Metz will continue as the head of schools.

In a statement, the Chicago Teachers Union said Milkie’s resignation was a victory for students who had “suffered” under “extreme disciplinary policies” and fines for minor infractions, such as dress code violations.

The network, however, has been praised for building a strong college-going culture. Noble is 98 percent African-American and Latino, and in 2016, the most recent year for which four-year college data is available, 80 percent of Noble graduates matriculated directly to four-year colleges, compared with 42 percent for Chicago schools’ students overall.

Read the letter from Noble’s board about Milkie’s departure below.