U.S. retail receipts rose modestly in the week, as back-to-school sales began in earnest, a trade group said Tuesday. The International Council of Shopping Centers said sales rose 1 percent in the week and climbed 3.3 percent compared to the same week of 2011. Sales were helped by a slight break from a harsh heat wave, the trade group said. Weather Trends International said the U.S. average temperature was 1.4 degrees Fahrenheit lower than the same week last year -- although it was 2.9 degrees warmer than the long-term average. Cooler temperatures arrived later in the week. By Friday, the heat wave had broken in the Northeast, just in time for weekend shopping. Retailers have been encouraged by lower gasoline prices, which increases discretionary spending. Gas prices, however, have turned higher, posting a third consecutive week of increases, the group said. The Energy Information Administration said a gallon of gasoline averaged $3.494 per gallon on July 23, up 6.7 cents from the previous week.