Early last week, the national StudentsFirst group gave Minnesota a "D" on education-reform efforts. Later in the week, the Education Week publication issued its annual grading system and gave the state a "C." And the last grade Minnesota received from the American Legislative Exchange Council in 2012 was a respectable "B+."

So what are Minnesotans to make of these and other education ranking reports? How can the public use such wide-ranging grades to assess how state schools are doing?

In evaluating the various grades, consumers must do what neutral education experts and journalists do: Consider the sources. There are hundreds of education advocacy groups out there, including unions, professional associations, business groups, foundations and parent groups -- and many of them issue school report cards.

That makes it crucial to look at each group's mission, what data they collect and how they use it before determining the meaning of a particular grade.

StudentsFirst is a newly formed national group, started by former Washington, D.C., education chancellor Michelle Rhee, a hard-charging, controversial reformer. The group advocates mayoral control of schools and voucher programs. So it gave Minnesota low grades in part for not having statutes in those two areas. Neither of those ideas have much support in polls of state citizens.

StudentsFirst gave its top "B" scores to Florida and Louisiana for recently adopted policies. But neither state yet achieves student outcomes as good as Minnesota's.

Education Week is a well-respected publication known for being more neutral and for conducting unbiased research. It has been compiling an annual "Quality Counts" state education rating system for 17 years; its results are widely followed and quoted. In its recent report, Maryland and Massachusetts came in first and second with B+. Minnesota ranked 39th with a C, and South Dakota came in last with a D-.

Ed Week grades states in a wider variety of areas -- six categories that include using research-based techniques for success; aligning grade levels with student progress; school finance; K-12 achievement; standards, assessments and accountability, and teacher quality. In addition, the study includes an interactive feature that allows readers to use their own priorities to recalculate the grades.

While one can quibble with details and overall ratings, last week's reports identify areas that need attention in Minnesota's K-12 system. StudentsFirst, for example, promotes the worthy idea of changing teacher seniority rules.

Last year, this Editorial Board supported proposed legislation that would have modified those rules. The bill would have scrapped the "last in, first out" system used during teacher layoffs, replacing it with a system that considers seniority as only one of several criteria for deciding which teachers to retain.

That bill was vetoed by Gov. Mark Dayton, but reform in this area should be considered again.

Independent assessments and grades can be useful tools in identifying the strengths and weaknesses of state K-12 programs. But the public should carefully consider the grading criteria and the organizations' agendas in evaluating state report cards.