NEW YORK—Strong late-summer back-to-school sales could provide U.S. retailers some needed momentum after many sector shares lagged the broader market in recent months. The S&P 500 retailing index, which includes Amazon.com Inc, is up just about 2 percent for the quarter so far, compared with a 6 percent gain in the S&P 500. Some retailers, including Macy’s Inc and Walmart Inc, cited upbeat back-to-school data recently as they increased annual U.S. sales forecasts, but further sales data and the U.S. August retail report are expected later this month. With the help of advance child tax credits and stimulus checks related to the pandemic, some consumers have had extra cash to load up on backpacks and other supplies. Most U.S. schools are returning this year to in-class instruction after months of remote learning. New York City’s public schools reopen Sept. 13, and many New York metro area schools begin in the …