The midmorning bell at Technical Secondary School No. 14 sets off a rush for the campus snack bar, where students jostle for chocolate bars, chili-flavored lollipops, packaged popcorn and sweetened fruit drinks.
There's yogurt and bottled water, but it doesn't seem much in demand.
"Potato chips taste great," enthuses Daniel Cuevas, 13, clutching a bag of chips and a bottle of orange drink.
After class, youngsters are greeted by a phalanx of sidewalk vendors hawking jelly-filled cakes, ice cream, mayonnaise-slathered ears of corn, french fries topped with cheese and cans of soda to wash it down.
Then they head home for midafternoon lunch, the main meal of the day in Mexico.
Mexican health officials say overeating and poor dietary habits have contributed to a fast-rising childhood obesity rate, behind only the United States for highest in the world. One-quarter of all school-age Mexicans, and more than one-third of adolescents, are overweight or obese. (More than two-thirds of grownups are too heavy.)
Mexico, whose biggest food worry used to be hunger, finds itself in a quandary over how to get its children to lay off the chicharrones and exercise more. It turns out that those two goals are more complicated than they sound.
The lower house of Mexico's Congress ignited debate last week when it passed legislation aimed at reducing the amount of nonnutritious food sold in schools. A related measure would require schools to ensure that pupils get 30 minutes of exercise each day.
The reforms, relayed to the Senate, led some newspapers to crow that junk food, known here as comida chatarra, had been expelled from school.
Critics said the measure wouldn't ban anything; it merely calls on health officials to establish "official Mexican norms" for student well-being and ensure that food sold in schools provides "greater nutritional content" and avoids excessive fat and sugar.
Other consumer advocates charged that lawmakers shied away from taking on soft-drink and snack-food giants such as Coca-Cola and Sabritas, a subsidiary of PepsiCo that makes potato chips and other snack products.
Complicating matters, federal education officials said it would be next to impossible to provide Mexico's 25 million schoolchildren with 30 minutes of daily exercise because space at most public schools is tight and the day is already short. Public elementary schools are in session just 4 1/2 hours a day, and 30 minutes of that is already taken up by recess.
"If we devote another half hour solely to physical activity, then we have 3 1/2 hours," Education Secretary Alonso Lujambio said in a recent radio interview. "We won't be able to complete the curriculum."
Then too, the politically powerful national teachers union blasted the exercise requirement as "authoritarian."
Obesity in Mexico hit worrisome levels years ago. But this is the first time it has been the focus of much attention by policymakers, who worry about the related risks, and social costs, of diabetes, hypertension and heart disease.
Health experts attribute the growing number of overweight young Mexicans to more urban living, an onslaught of U.S.-style snack products and marketing, shifting tastes and a sedentary lifestyle that centers on television and video games more than the playground.
A health ministry report issued in January found that only 35% of Mexicans from ages 10 to 19 are physically active. One in four adolescents watches at least three hours of TV a day. (The same study also noted said the average pupil has five chances to eat during the brief school day.)
Like Americans, Mexicans are often too busy at their jobs to cook well-balanced meals from scratch, relying instead on packaged products.
Some schools in Mexico City are testing health-conscious food offerings, while others are waiting for guidance from health officials on what products should be barred at campus snack outlets, known as cooperativas.
School administrators say it's up to parents to teach good nutrition, but it can be awfully hard to defeat so much temptation.
Rodrigo Fernandez, a plump 9-year-old, stood outside his school on a recent day and tallied his intake. He had begun his day with a breakfast of milk and bread with jam. During school, he'd eaten two tacos filled with rice and sliced hot dogs and chugged a packaged mango drink. Now, shortly after noon, he waited in line at a stand to buy fries.
His mother had sent him to school with an orange. Rodrigo didn't eat it.
Cecilia Sánchez of The Times Mexico City Bureau contributed to this report.