Back-to-school sales disrupted by Amazon Prime Day

David Paul Morris | Bloomberg | Getty Images

Customers shop for back to school supplies at a Target Corp. store in Colma, California.

It's August, which usually means summer comes to an end and back-to-school shopping heats up — but maybe not so much this year.

Shoppers have been hitting the shelves for a month already thanks to Amazon's Prime Day, and retailers need to step up their game to keep their spot in the back-to-school shopping season.

New data provided to CNBC show that the school shopping season — second in importance only to the holidays — is starting earlier and not running as late into the year, thanks to Prime Day in July, and other retailers following suit. Specifically, the online sale was the peak of back to school in 2015, according to Cardlytics, a firm that tracks purchasing data. No other week of shopping during July and August came close last year.

Amazon introduced Prime Day in 2015 to boost sales during a traditionally slow part of the year. Positioned as a celebration of Amazon's first 20 years, consumers were disappointed by the deals. Still, sales volume exceeded Black Friday sales from 2014. In 2016, the response was even stronger, with a 60-percent bump in sales over 2015, according to researchers at Piper Jaffray.

In 2016, other major retailers wanted in. A lot of other retailers tried to retaliate against the online giant with their own "Black Friday in July" promotions, as Macy's called it.

"Back-to-school season is second only to holiday in terms of consumer spending," said Dani Cushion, chief marketing officer at Cardlytics. "Retailers can take advantage of the increased traffic to enroll consumers in loyalty programs that will drive continued purchases year round."

The data suggest that shoppers got a lot of their online back-to-school shopping done early in the season. Sales appear to have petered out in the beginning of September. That goes against the trend we've seen in the past few years of a big bump in apparel shopping after the school year actually starts. CNBC reported last year on the "clothing bump" — parents hitting the stores again when their kids have seen what styles are cool.

The data show that overall, shoppers are moving ever more to shopping online: Brick-and-mortar stores saw their back-to-school revenue fall 2.4 percent in 2015 from 2014. The number of purchases also dropped by 3.4 percent, the data show. Online-only stores saw 14.5 percent greater revenue and 18.9 percent more purchases in 2015.

The equivalent figures for 2016 naturally won't be available until the back-to-school season is over, but considering the success of Prime Day this year — and how many other retailers hopped on the bandwagon — it's likely the trend has continued.

"Back-to-school spend is shifting forward with events like Amazon Prime Day and the continued adoption of year-round school schedules," Cushion said. "Because of this, retailers should consider starting their back-to-school campaigns early to capture more of that spend."