All over the United States politicians face a dilemma: They need to pay for rising school costs without raising taxes. For example, when the federal government imposes unfunded mandates on local school systems, such as the $35 billion unfunded portion of the No Child Left Behind Act, the money must come from somewhere. Often at least part of the money comes from tolerating -- or even encouraging -- depressed public school enrollments.

Kids enrolled in public schools are costly for local governments. The average K-12 public school student costs taxpayers close to $8,000 per year. Over a 13-year public school career, that comes to more than $100,000. Raising taxes to pay for rising enrollments is a political nightmare, as is redistricting students out of crowded but desirable neighborhood schools.

If you assume that there just isn't any way to ask taxpayers to pay more for schools, then you have no choice: You have to find a way to keep fewer kids enrolled in public schools. Whether they're aware of it or not, U.S. politicians are doing just that in three ways. First, they're discouraging development that includes family dwellings. Of course, the Federal Housing Act prohibits discrimination against people with children, so this must be done indirectly. Tactics include discouraging new residential construction, encouraging office and retail development, ensuring that single-family homes will be built on large lots subject to strict building codes (this increases property values and hence taxes per household with kids) and allowing high-density residential construction only for single- or double-bedroom houses that are designed for households that don't have children in public schools.

A second way to keep costs down is to facilitate high dropout rates among poor, at-risk students. Recently, there has been a lot of talk about the nefarious use of high dropout rates to keep test scores high (the students who drop out test poorly). Achieving improved test scores at at-risk schools is a centerpiece of the No Child Left Behind Act and a key requirement for keeping federal Title I money flowing to these schools. But high dropout rates also have another effect: They keep down student enrollments. If dropout rates suddenly decreased, many school districts would have a financial and facilities crisis. No one's objecting too loudly about this loss, because there is neither the money nor the space to keep these students in the public schools.

In a similar way, tolerating high dropout rates among students of affluent families also can help politicians solve their tax dilemma. In Maryland, for example, public school enrollment increased by just 17 percent over the last decade, while private school enrollment increased by 36 percent. In my own school system in Maryland, Anne Arundel County (population 490,000), the corresponding figures are 13 percent for public and 58 percent for private school enrollment growth. For the past five years, the county's figures are 2 percent public vs. 23 percent private school growth. The numbers would be even worse, but the good private schools are at capacity and cannot build fast enough to meet the demand. Much of the public school flight is concentrated in small, affluent communities such as Severna Park and parts of Annapolis.

Public officials aren't the only ones at fault. Affluent families help cause the problem because they don't want to fund schools their children don't use. As public schools fall behind private schools in class size and other indicators, those parents with financial options exercise them. Until the 1970s, private schools had larger pupil-teacher ratios than public schools.

Yet public schools also are at fault, because they have developed curriculums and priorities that essentially say good riddance to those who can afford to leave the system. Every student who leaves means more dollars for those who remain. Instead of building new schools to meet rising enrollment, for example, money can be spent repairing dilapidated schools.

The result of the increase in private school enrollments is that public schools are being turned into a welfare service for the disadvantaged. A vicious cycle is being established whereby students at the margins are being driven out, which meets short-term budget needs and keeps most of those left in the system happy. But those who are safe now become at risk for the next round of enrollment cuts.

Of course, no one acknowledges this is going on, and many of the most active players bringing about this transformation have done so as an unintended consequence of their understandable priority to allocate scarce educational resources to those with greatest needs and fewest options. For example, the current shift to a back-to-basics curriculum to improve pass rates on reading and mathematics tests may have the effect of turning off ambitious parents who want their children to attend competitive colleges. These colleges expect students they admit to have a well-rounded education.

In the long-term, the transformation of the public schools into a welfare service is a disaster for the United States. Today local governments typically spend close to 50 percent of their resources on public schools. If public schools turn into a welfare service, that percentage will surely plummet.

There are no easy solutions. But as a start, I suggest that public school advocates stop supporting the legions of politicians who give lip service to excellent public schools and then send their own children to private schools. Politicians need to stop viewing the public schools as a place for other people's children; that is, as a welfare service. Unless the politicians have a bone in this fight, they aren't worthy of our trust.