Public schools are like politicians when it comes to public opinion. As a whole, they can be the object of broad, negative generalizations. But if you ask people about their own children’s schools — or their local member of Congress — it’s a different story.

Consider the words of Mark Twain. “Suppose you were an idiot. And suppose you were a member of Congress,” Twain quipped. “But then I repeat myself.”

Twain’s clever jab is one thing, but would you tell Rep. Richard Hudson or Rep. Ted Budd that you think he’s an idiot? More important, would you believe it? Probably not. People generally respect their own members of Congress, party differences aside.

So let’s talk about the nation’s schools.

President Trump and a good many other people say our schools are “failing.” Within conservative circles, public education has become the whipping boy for society’s ills — one of those institutions outsiders regard with disdain.

A Phi Delta Kappa survey of attitudes toward public schools found that only about one in four people polled gave the nation’s public schools an A or B grade.

The closer respondents are to actual schools, though, the higher their opinion of them.

In fact, when the PDK survey asked people how they would grade the school attended by their oldest child, the response was overwhelmingly positive. Nearly three-fourths — 71 percent — gave the school an A or a B.

“In other words,” educator Elaine Weiss writes in an analysis of survey results, “the more you see for yourself what goes on in your child’s school and what the teachers, nurses, counselors and principals who work there do every day, the more strongly you support it.”

Maybe that’s why Superintendent Dr. Lynn Moody offered a suggestion for critics of the Rowan-Salisbury School System last week: If you haven’t visited the schools for some time, don’t write a letter to the editor.

That is, don’t present yourself to the public as an expert on schools if you haven’t even been in one.

Moody was speaking at a Rowan County Chamber of Commerce breakfast, explaining to community leaders the fallacy of judging local schools solely by state test results.

Rowan-Salisbury made modest progress in the last round. Four schools improved their letter grade and Rowan Early College scored the county’s first A, but two more schools received an F.

Moody didn’t make excuses. But she said there’s much more to education than producing good test-takers.

Tests are needed, make no mistake. Being able to do double-digit multiplication in third grade is a good standard, Moody said. But those criteria don’t tell the whole story.

She referred to a pendulum swinging back and forth. At the extreme reached a couple of decades ago, there was little standardized testing shared with the outside world. Teachers could set their own criteria for success.

Now we’re at the other end of the pendulum’s swing, with a state-mandated bureaucracy labeling the entire school with a single letter grade to be worn like a badge of success or sign of failure, A through F.

But the grades tend to reflect something else: community wealth, or the lack of it. It’s unlikely you’ll find many high-poverty communities represented in the 181 N.C. schools that received an A — and even more unlikely that you’ll find wealthy communities among the 98 schools that received an F.

“Poverty is not somewhat related to test scores,” Moody said. “It is exactly related to test scores.”

The state’s grading system puts much more weight on a student’s achievement than how much that student improved. Moody says people who want to know how well the schools are doing should look at growth, not just mastery.

Talking as he walked to his car afterward, state Rep. Harry Warren was willing to say the state might not have found the perfect way to measure school performance. But, he said, the state had to have some way to hold educators accountable for the job they do.

Sure it does. But do these letter grades really reflect school performance — how well the teachers and principal and staff do their jobs?

Or does the grading system simply guide some parents to the “better” neighborhoods  and send others running into the arms of private schools?

The premise of the grading system is that all schools receive the same raw materials — the same students, with the same skills — and testing shows how well the administrators and teachers do their jobs. But schools and their student bodies are more like snowflakes than snowflakes. The variables are endless.

The state needs to refine its accountability program — without increasing paperwork or bureaucracy.

Is Moody discouraged by the public criticism that followed recent test sores? “Actually,” she said, “we do our best under pressure.”

In that case, 2017-18 should be the best year yet.

Elizabeth Cook is editor of the Salisbury Post.