The average expenditure for apparel, shoes, electronics and supplies will fall to $634.78 from last year's record $688.62, which was partly driven up by pent-up post-recession demand, according to the trade group.
In all, parents will shell out $26.7 billion nationwide throughout the season. Separately, preparations for college will cost Americans $45.8 billion, or $836.83 on average.
The per-student college figure is down from $907.22 last year and includes spending on clothing, technology, dorm furnishings and more.
Roughly 8 in 10 shoppers - whether buying for primary, secondary or higher levels of education - say they'll adjust their spending plans to deal with the economy.
"As they continue to grapple with the impact of increased payroll taxes, Americans will look to cut corners where they can, but will buy what their kids need," said Matthew Shay, the retail group's chief executive, in a statement.
Parents shopping for school-age children are getting an early start, with 23.9 percent of families saying they are already scouring the racks and nosing through shelves. That's the highest percentage ever of consumers launching their back-to-school preparations two months before classes begin.
The ratio of early birds on the college shopping circuit also hit a record, with 29.8 percent looking for deals now, according to the report.
More than a third of parents say they plan to do more comparative shopping online. Nearly 4 in 10 college shoppers say they'll hit the Internet for their retail needs.