By Angela Johnson

New York (CNN Money) - As children across the country start getting ready to say goodbye to summer, 17 states are preparing to offer shoppers tax breaks on back-to-school items.

The tax savings could amount to anywhere from 4% to 7% on everything from crayons to computers.

That savings could come in handy. Economic uncertainty, unemployment and a recent surge in gas prices are forcing parents to focus on necessities this school year, says Matthew Shay, chief executive of the National Retail Federation. Still, families with school-aged children are expected to spend an average of $635 on apparel, shoes, supplies and electronics during this year's back-to-school shopping season, down from $688 last year, the industry trade group found.

Before heading to the stores, shoppers in the states where these temporary breaks are being offered should research which items are tax exempt and the restrictions that apply, said Carol Kokinis-Graves, senior state tax analyst at CCH, a global provider of accounting and audit information.

In Florida, for example, clothing that costs less than $75 qualifies. But any item that costs more than that amount does not. Want a personal computer? You can get a tax break in Florida, but only if you opt for something that costs less than $750 - not that MacBook Air you may have been eying.

If you gotta' have that top-end Mac, try Missouri or North Carolina; those two states are offering breaks on computers worth up to $3,500.

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