With Minneapolis school superintendent Bernadeia Johnson stepping down from the job as of Jan. 31, the school board must step up at a critical time for the district.

The board ultimately must select a new superintendent who can expedite improvement in student learning, and board members must have the courage to support their pick if some needed changes are not universally popular.

Johnson's decision to leave in the middle of a school year has been described as "abrupt'' and "surprising.'' She released a statement saying that she was leaving after four years on the job to help care for elderly grandparents. But there also has been speculation that Johnson lacked broad support on the board and was concerned that her contract would not be renewed in 2016.

Responsibility for selecting a new school chief and guiding the district through another leadership transition lies with the board's nine members, five of whom will start terms in January after winning November elections. Three of the five are newcomers; eight of nine have not been involved in a superintendent search before.

Johnson's short stint in the top job is not unusual for superintendents in larger cities. According to the Council of Great City Schools, the average urban school chief serves for 3.6 years. Frequent turnover has become more the rule than the exception nationally, as superintendents have struggled to balance politics with improving outcomes among growing numbers of lower-income students of color.

As we pointed out when Johnson was hired four years ago, those numbers are revealing and unsettling. Minneapolis has had more than its share of that kind of disruption. Over the past 30 years, the district has had 14 superintendents, including interim chiefs.

Not surprisingly, research shows a correlation between longevity and academic achievement. District leaders who focus on a few key goals, manage change effectively and stay with their districts long enough to see results tend to have higher-performing students.

As for Johnson's legacy, no one has questioned her commitment to improving student learning. She has always shown passion for the work, whether serving as a teacher, principal or superintendent. In recent years, she has repeatedly emphasized "urgency,'' and has sometimes become emotional about the challenges facing many students.

Johnson set measurable benchmarks for student success. And in her "shift'' speech 18 months ago, she made it clear that the district's staff needed to do some things differently if they wanted different results. Instead, the district's achievement gap grew.

There were plenty of missteps during Johnson's tenure, including her decision to give pay raises to administrators about a month after the board voted to lay off 118 employees. But there were also successes, such as building strong relationships with the business and nonprofit communities.

Johnson also deserves credit for developing partnerships with successful charter programs. While leaders in other districts have viewed charters as competitors, Johnson sought to collaborate with them and put their best practices to work in traditional public schools.

It's also worth noting that the Minneapolis district continues to post positive results for middle- and higher-income students. Unlike in some urban centers, many higher-performing school communities have strong parental support and involvement. The challenge for the board is to find a leader who can implement effective strategies for low achievers, while remaining attractive to a full range of students.

Johnson, 55, said her family responsibilities would not allow her to approach her work with the "level of intensity and focus" that's needed to make that happen. That's understandable, and Johnson deserves credit for knowing it was time to step down.

One district observer described the departing superintendent as a "change agent in a system that is resistant to change.'' The next school chief must also be a change agent — and one who has the staying power to produce lasting results.