CHICAGO – The sandwich as a school-lunch standby has grown stale.

Rather, snack foods, such as granola bars and fresh fruit, are subbing in for the traditional P.B.&J. in children’s midday meals, said Harry Balzer, chief industry analyst for the NPD Group, a Chicago-based market research firm, and author of “Eating Patterns in America.”

Click the infographic to see the top snack foods consumed in schools.

“We think Americans are snacking more, but they aren’t,” Mr. Balzer said. “Certain snack foods are just making their way into the main meal. Snack foods by definition are ready to eat and rarely require preparation.”

And preparation equals time, one of the largest drivers behind meal selection, Mr. Balzer said. While sandwiches remain the top school-lunch entree, the staple has declined over the years, from being included in 73% of school lunches brought from home by 6- to 12-year-olds in 1995 to 58% in 2012.

Meanwhile, packaged lunch kits are on the rise, and more fruit and fruit cups, cracker products and yogurt are being toted to school, Mr. Balzer said. Chips and cookies, he added, have been bumped from the brown bag.

The top snacks eaten at school vary by age group. Teenagers are more likely to consume gum and chocolate bars between classes than 6- to 12-year-olds, who tend to eat healthier fare packed by their parents, such as fruit, applesauce and yogurt.