K12 Inc. /zigman2/quotes/205813628/composite LRN -3.85% announced Tuesday that Nate Davis will be chief executive again, replacing Stuart Udell, who resigned after apparently refusing to accept new responsibilities. Davis was previously CEO of K12 from 2014 to 2016, when Udell was selected to replace him in the role, and was acting as chairman of the board. According to Tuesday's announcement, the board decided to "redefine [Udell's] responsibilities," and Udell resigned in response. "Our board has asked me to focus on a long term acquisition and business development strategy that puts our cash to use improving shareholder returns," Davis said. K12 has been the focus of regulators in California , where a 2016 Mercury News expose showed K12 was exerting more control over charter schools that used its services than their nonprofit status required. K12 stock has fallen 14.4% in the past year, as the S&P 500 index /zigman2/quotes/210599714/realtime SPX -1.12% has gained 14.1%.