When one door closes, another one opens, the adage goes.

That’s certainly the case with Eric Knost’s upcoming departure as superintendent of the Mehlville School District. Knost, who becomes superintendent of the Rockwood School District on July 1, has performed a yeoman’s job in Mehlville.

When Knost became superintendent on July 1, 2011, he had his work cut out for him.

Perhaps Venki Palamand put it best when he was elected board president in April 2011 and said the Board of Education’s top priority should be restoring the credibility of the Mehlville School District.

That credibility was severely damaged by failed former Board of Education members who lost sight of the principles on which they originally ran for the board and were blinded by their personal quest for glory.

Fortunately for Mehlville, Knost was up to the challenge and his successful superintendency is marked by much forward motion for the district and its students — accolades for academic achievement, character-education honors and construction of the district’s first-ever auditorium, to name a few of those successes.

We particularly were impressed with Knost’s commitment to transparency and his willingness to meet the public, including district critics, at his popular town-hall meetings.

But as the door closes on Knost’s tenure at Mehlville, another door opens with the Board of Education’s decision last week to name Norm Ridder the district’s interim superintendent, effective July 1.

Ridder, who now heads the largest school district in Missouri, comes to Mehlville with more than 40 years of teaching and administration experience in Nebraska, Colorado and Missouri. Since 2005, Ridder has led Springfield Public Schools, which has 25,000 students and 53 schools.

A Nebraska native, Ridder notified the Springfield Board of Education last year that he was retiring as superintendent at the end of the 2013-2014 school year.

Under Ridder’s guidance, Springfield’s graduation rate has increased from 75 percent to 88.7 percent, attendance has gone to the highest level in 20 years and the dropout rate has dropped to half what it was before his arrival.

We applaud the Board of Education for its decision to name Ridder the district’s superintendent, and we wish Ridder the same success here that he enjoyed in Springfield.