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You know how聽to make聽the grade in school: Do the homework and study the course material. But getting straight A鈥檚 in聽stocking up on school supplies聽doesn鈥檛 exactly come with a rulebook.

That鈥檚 why we made one. We have the when, where and how of聽shopping for the 2016 back-to-school season 鈥� saving money while you鈥檙e at it.

Pinpoint when to buy

If you鈥檝e been waiting for the right time to start shopping, now鈥檚 your chance.

聽in 17 states across the country in 2016. These limited-time events often fall in August,聽and they聽give shoppers a break on sales tax in certain school-oriented product categories, such as apparel and electronics.

But even if your state doesn鈥檛 participate 鈥� or you missed it聽鈥� expect back-to-school sales to continue throughout August and into September. In聽聽retailers offer聽deep discounts on writing utensils, backpacks and more.

That鈥檚 welcome news. Families with children in grades K-12 plan to spend an average of about $670 on 2016 back-to-school shopping,.

If sales don鈥檛 impact enough of your purchases, and you can鈥檛 afford to buy everything聽at once, spread it聽out.听Students generally need the essentials 鈥� a backpack and a lunch bag 鈥� on the first day of school, but some items聽鈥� markers and glue 鈥� can wait. Make less crucial purchases聽in聽September, if necessary.

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Map out where to shop

For 55%聽of Americans,聽price is the most influential factor in deciding whether to buy specific back-to-school items,聽.

To get the best deal,聽you have to know where to find聽sales. We rounded up some of the stores with the best back-to-school聽discounts, including聽budget supplies and聽generous price-matching policies.

:聽If your money is going toward聽summer vacation and school spending, you might not be able to check off the entire supply list right now. Kmart is offering to help with no-penny-down in-store layaway. This offer is valid on new layaway contracts through Aug. 20. Some exclusions may apply. If you choose online layaway, you鈥檒l need to put one penny down.

:聽Office supply store Staples won鈥檛 just match competitors鈥� prices 鈥� the retailer promises to give you 鈥渆xtra credit,鈥� too. Through Sept. 17, prove a qualifying competitor has聽a lower price on an identical item, and you鈥檒l get the lower price plus 10% of the difference. To redeem,聽bring proof of a qualifying lower price聽to the store or聽call 800-333-3330.

:聽School supplies start at just 99 cents. The store also matches the prices online competitors charge for聽items聽such as laptops and backpacks. Competitors include Amazon.com, BestBuy.com and Costco.com.

:聽Back-to-school shoppers can buy more than聽300 classroom essentials for less than $1, more than 600 items for less than聽$2 and more than 1,000 items for less than $3, according to a store spokesperson. For instance, a聽聽will run you just 50 cents.

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Implement savings techniques

You know where and when to buy, but how else can you save? Here are NerdWallet鈥檚 best school shopping tips:

• Find the best price.听Compare prices from multiple retailers by conducting a Google search or using a tool like聽. In most cases, you鈥檒l need proof of a competitor鈥檚 price if you want to request a price match.
• Do the math.听Calculate the price per unit to make sure you鈥檙e getting the best deal, even when items are sold in different amounts. Paying $6 for a聽聽(30 cents per pen), for instance, is better than paying $4 for a聽聽($1 per pen).
• Know the return policy.听Check聽the store鈥檚 return policy before you make major purchases, such as聽laptops and tablets. It鈥檚 usually listed on the website. Look for聽the length of the return window and any items that聽are final sale or聽non-returnable.
• Buy online, pick up in store.听Many retailers give聽their shoppers incentives to buy online and pick up their items in store. Some offer free shipping to your local store 鈥� including聽听补苍诲听鈥斅爓hile others discount your order or, like聽, provide聽a coupon for next time.
• Shop with a credit card.听Using聽聽for back-to-school shopping can boost your savings, thanks to promotional deals and cash-back offers. Some give bonus points for聽purchases at department stores or wholesale clubs.

Courtney Jespersen is a staff writer at NerdWallet, a personal finance website. Email:聽courtney@nerdwallet.com. Twitter:聽.

This article first appeared in .听