Now that John McCain has agreed to join tonight’s presidential debate in Mississippi, it seems likely that the economy could usurp the scheduled topic of foreign policy as the main focus of the questions.

Although the issue of education isn’t likely to come up with much significance, there’s plenty reason for those interested in public schools to listen carefully to the presidential candidates when they talk about the economy. And that’s because, just like so many parts of our lives, the health of the economy has a dramatic effect on school districts.

For one, school districts need the credit market to help pay for their construction projects. This week, as I was reporting a story about how the financial meltdown is affecting the bond market, I discovered the plight of Laurens County School District 56 in South Carolina, which is trying to finish building a new high school. Superintendent Wayne Brazell told me in an email that his district had to postpone selling its final, $28 million in bonds this week because of the upheaval in the credit markets. The district will try again on Oct. 1.

Secondly, the ups and downs of the stock market affect the investment portfolios of teacher pension funds, school districts’ savings accounts, and individual teachers’ supplemental retirement accounts.

And lastly, and perhaps most important, the overall health of the economy has a trickle-down effect on the amount of money school districts receive from local, state, and federal sources. Home foreclosures that are hitting some markets hard will threaten the stream of property taxes flowing into districts—money that’s used to pay the day-to-day operations of many schools and pay off district debt. The sagging economy has depressed tax revenues coming into states, which has already threatened public school funding in some states.

Stay tuned to Campaign K-12 for coverage of tonight’s debate, which is scheduled to start tonight at 9 p.m. EDT.