See-through backpacks and jean shorts are among the popular last-minute purchases ahead of the new school year, several retailers said, as heat waves sweep the U.S. and security concerns grow at many middle- and high schools. Though U.S. student debt repayments will resume on Oct. 1, straining many Americans' budgets, 2023 is expected to be the most expensive back-to-school shopping season yet. The National Retail Federation, a retail trade group, predicts spending will surpass $135 billion, an increase of more than $24 billion from last year.