This week, thousands of parents in Prince George's County will receive a letter saying that their children have been taught this year by a teacher who is not highly qualified.

If you are one of those parents, take a deep breath and don't panic. Walk, do not run, to your child's school.

But, before you do, let me fill you in a little on the background of these letters, because they are a sign that schools are changing. I hope they are changing for the better, but they are definitely changing.

Time was when a teacher's credentials were considered almost private information. Although some teachers happily shared details of their background -- education, degrees, certification and experience -- others kept quiet. Still others treated questions about their credentials as insults. Many principals fended off questions from parents about teacher credentials with vague statements that all the school's teachers were equally qualified.

Some of this defensiveness and vagueness came from a desire to hide the fact that some teachers were completely unqualified to be in a classroom. But another factor was an idea in education circles that the quality of teachers was unimportant. Schools and teachers don't affect students nearly as much as parents and families do, the argument went, so it really didn't matter who was teaching.

A great deal of research has demonstrated conclusively that teachers do matter. I call this "well, duh" research, because it demonstrates what common sense would tell you if you thought about the issue for two minutes. But with common sense in short supply, such research plays an important role. In this case, it has the potential to transform schools.

In Tennessee, for example, careful analysis of data collected by the state on individual students has shown that in some classrooms, children learn a lot more than in others -- pointing to a wide range of teacher effectiveness. In Texas, research has demonstrated that poor children who are assigned five highly effective teachers consecutively achieve at levels that are indistinguishable from well-to-do children.

This and similar research are why the federal No Child Left Behind law makes a big deal of teacher quality. As of last year, no teacher could be hired to teach in a Title I school without being "highly qualified." (Title I schools are those that receive federal money because they have large numbers of children living in poverty.)

By the 2005-06 school year, every classroom in every school system is supposed to be staffed by a highly qualified teacher. In the meantime, No Child Left Behind requires that all parents in Title I schools be told that they have the right to know the qualifications of their child's teachers -- and to be informed if their child has a teacher who is not highly qualified for longer than four weeks. That's the reason for those letters I was talking about.

Clearly, the letters should have gone out long before this, considering that we're now almost at the end of the school year. But this requirement is a very complicated change in procedure. Next year, the letters should go out much earlier -- by the end of September.

To give a sense of the complexities, in Prince George's there are approximately 10,000 classes in Title I schools, in Grades 3 through 8, being taught by teachers who are not considered "highly qualified," according to Leroy Tompkins, the school system's chief accountability officer. One elementary teacher who is not "highly qualified" could be teaching English, math, social studies and science, and thus could represent four of those classes.

Conversely, a middle school teacher who is highly qualified to teach math might not be considered highly qualified for the one science class he or she is teaching, so letters would go out to the parents of students in the science class but not the math class.

No high school classes were counted because no high schools in Prince George's are Title I schools.

Keep in mind that in applying this new category "highly qualified," each state has its own standards. In Maryland, the term means that a teacher not only is certified but also has demonstrated mastery of the particular subject he or she is teaching, either by having successfully completed college course work in the subject or by having taken a test in the subject.

Because the concept is so new, many teachers who would be considered fabulous by almost anyone may not yet meet the criteria for "highly qualified." For example, a math teacher of my acquaintance who has a degree in microbiology from the University of Chicago is not "highly qualified" because the results of her math test have not yet come back. Once they do, I am confident she will be considered "highly qualified," but until then, she is not.

That's why parents should use those letters as steppingstones to conversation, not to panic. Teachers should be happy to discuss what they know and why they are qualified for the jobs they have. Principals should do the same.

Beyond the question of credentials, parents should ask for evidence that their children have been learning and progressing this year -- that their reading is improving, that they know more math and so forth.

This raises an issue concerning the term "highly qualified" that parents should be aware of: That there really is no evidence that the kinds of qualifications Maryland requires guarantee a good teacher.

Certainly the intention of the requirements is that a teacher know the subject he or she is teaching -- another "well, duh" concept that got lost for any number of years. But lots of people who know a lot couldn't teach their way out of a paper bag. A PhD in physics doesn't necessarily mean a person will be good at teaching middle school science.

The standards for "highly qualified" are really a rather crude proxy for trying to figure out which teachers are effective and which aren't. There's a much more direct way to do that -- by measuring what kids know when they enter a classroom and what they know when they leave. That gives a sense of how effective the kids' teachers are.

Tompkins is working to create that kind of system. "We're creating some rich databases to help us develop it," he said. "Without question next year," he said, principals will be able to see the effectiveness data on each of their teachers.

Parents won't be able to see that data, but if their child is learning below grade level or didn't progress much this year, they should ask their principal to ensure that their child is assigned a highly effective teacher next year. Most kids can survive one year of an ineffective teacher, but two ineffective teachers in a row is very difficult for a kid to overcome without a lot of specialized help.

Homeroom, which appears every other week, is a forum for you. Send questions, opinions and issues that you would like to see discussed to Homeroom, The Washington Post, Prince George's Extra, 9500 Arena Dr., Suite 400, Largo, Md. 20774. The fax number is 301-618-1780; the e-mail address is homeroom@washpost.com.