Minneapolis Schools' superintendent Bernadeia Johnson took part in the swearing in of new school board members in Minneapolis, Minn. Tuesday, Jan. 11, 2011. Jeffrey Thompson / MPR News 2011

Updated 8:36 a.m. Wednesday | Posted 6:16 p.m. Tuesday

Minneapolis Public Schools' superintendent Bernadeia Johnson is stepping down from the job she has held for more than four years, creating more turmoil in the district at a time when it faces tough challenges.

The Minneapolis School Board accepted Johnson's resignation at its meeting Tuesday. Johnson, who did not attend the meeting, plans to stay in her position through the end of January.

According to a statement on the district's website, Johnson said "family commitments have become increasingly challenging" and that "stepping down would best serve both the school district and her family."

"My commitments to family - specifically the care of elderly grandparents - are increasingly in competition with the extraordinary demands of this position," Johnson wrote. "Without question, the work of educating our children must move forward, and at this time, I must put my family first.

Minneapolis Public Schools CEO Michael Goar will serve as interim superintendent.

Board member Alberto Monserrate said the district's next superintendent would need to push forward with plans the district has put in place to raise student achievement.

"The challenge that this district has seen for many decades is a challenge of great plans and great initiatives, but not having those be implemented," Monserrate said. "One of the most important factors we're going to need in the next superintendent is very strong leadership and implementation skills."

Several school board members told MPR News that the board did not ask for Johnson's resignation. But they acknowledged her frustration at the lack of improvement in test scores may have played a role in the timing of the announcement.

School Board Chair Richard Mammen called her an exemplary leader and champion for children, schools and urban education.

"Dr. Johnson brought to our district an unwavering focus on the potential of the children of Minneapolis," he said. "Amidst all the strategic planning and the hard work of meeting the changing requirements for public schools, her primary attention was always on the students and the obligations we have to ensure their successful futures."

In a letter submitted to Mammen, Johnson said she continues to have strong confidence in the direction set for improving outcomes for Minneapolis students as set out in her Acceleration 2020 strategic plan. Earlier this year she said the blueprint contained 47 measures that the district will monitor over six years to eliminate the gap in test scores between white students and students of color.

They include erasing racial inequities in suspensions between white students and students of color, improving student attendance and the four-year graduation rate, boosting the effectiveness of principals and teachers and better engaging parents in their children's education.

Under Johnson's tenure, the district has struggled to meet those goals.

Although school district officials in recent years said they were pleased with an increase in the district's four-year graduation rate, Johnson acknowledged that overall rates are still too low.

In an April 2013 email to parents, Johnson noted that just over half of students in the district graduate in four years, 20 points lower than the statewide average.

Data from the Minnesota Department of Education in 2012 showed that only a quarter of American Indian students graduated in four years. The rate is 36.8 percent for both Latino and black students. Nearly 70 percent of Asian and white student graduated on time.

In September, the school district announced an ambitious goal to close that achievement gap by 2020.

As a part of the plan, Johnson said the district would give some schools intensive help to raise the achievement of struggling students, while other schools would receive more autonomy to try innovative approaches.

Johnson also has acknowledged that the racial imbalance of students who are being sent home for behavioral problems is so staggering that district leaders needed to rethink the standard for removing children from school.

The vast majority of students receiving suspensions are African-Americans, who are nearly seven times as likely as white students to be pulled from school. And the rates for American Indians are not much better.

Nearly 14 percent of African American students in the district were suspended in 2012, compared to just 2 percent of white students, according to district data.

"I don't think I can explain it," Johnson said in 2013. "I can tell you that it's disturbing, that it's concerning, and it's something that we must address."

Across the country, students of color are facing severe consequences for their behavior at higher rates than their white peers.

The imbalance was so troubling that in September, Johnson declared a moratorium on suspending very young students for non-violent behavior, a change that immediately applied to students in prekindergarten, kindergarten and first grade.

"Understanding and navigating the new rules and environment simply takes longer for some students than for others," she said. The move was news to teachers, who support reducing suspensions but were surprised when the ban was announced.

In November Johnson said the district would begin reviewing all suspensions of black, Hispanic and Native American students.

She said the effort would help the district better understand which students are suspended and why, and provide data that could help the district reduce suspensions in the future.

The plan is part of an agreement with the U.S. Department of Education's Office of Civil Rights that came after an investigation into the disproportionate number of African-American boys who were being suspended in the district.

District officials say suspensions in Minneapolis have begun to decline, with 50 fewer suspensions this fall compared to the same time last year.