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Surprise expenses that bust back-to-school budgets

Beating back-to-school budget busters

Parents may not get much of a budgetary breather between back-to-school and holiday shopping.

The average family with kids in grades K-12 is expected to spend about $670 on back-to-school shopping this year, according to the National Retail Federation. That total, up 5 percent from last year, covers a range of purchases from backpacks and pencils to jeans and sneakers.

Read More Expect to spend more on back-to-school this year: NRF

What's missing? Miscellaneous school fees, which can add up to hundreds of dollars—and often aren't negotiable. According to a 2012 University of Michigan study, 61 percent of middle and high school students paid a fee to participate in school sports. About one-fifth of those students paid at least $150. (And that doesn't count costs for equipment or travel.)

Not trying out for the team isn't necessarily a cost-saver, either. Some schools charge fees to participate in any after-school club, or for academic choices such as taking a lab science or an advanced-track course.

—By CNBC's Kelli B. Grant