The back-to-school shopping season is currently in full swing, and the average parent could spend an average of $221.24 per K-12 student this year, according to a recent Promocodes.com survey of 2,000 U.S. parents. Those parents also chose Walmart (NYSE:WMT) as their favorite back-to-school retailer.

Data source: Promocodes.com. Chart by author.

Parents likely favored Walmart and Target (NYSE:TGT) over Amazon (NASDAQ:AMZN) for three reasons: their prices were comparable to Amazon's; families could go together and try out products (like clothes and bags) first; and parents could finish all their purchases at one place and instantly take the products home.

Walmart, Target, and Amazon's dominance of the back-to-school season reflects their overall strength in the U.S. retail market. Walmart and Target both countered Amazon by turning their brick-and-mortar stores into fulfillment centers and pickup points for online orders, and all three companies now offer same-day deliveries.

Retailers that can't keep pace are struggling. That's why up to 12,000 stores across the U.S. could close this year, according to Coresight Research. This war between Walmart, Target, and Amazon will likely escalate into Black Friday -- and we'll see whether the superstores can maintain their edge against the e-commerce king.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Leo Sun owns shares of Amazon. The Motley Fool owns shares of and recommends Amazon. The Motley Fool has a disclosure policy.