One of the more popular “solutions” to the world’s obesity epidemic is to squeeze more exercise time into the school curriculum. It seems like a reasonable idea: more gym time, less fat. But a new study out of Britain says teachers should probably stick to reading, writing and arithmetic—because the extra running isn’t doing the fat kids any good. According to the lead researcher, youngsters who have a lot of physical activity during school hours tend to wilt when they get home, while children who have less action in school are more active after the final bell rings. Bottom line: at the end of the day, a child will expend the same amount of energy—and burn the same amount of calories—whether in school or out. Which brings us back to rule number one: don’t feed your kids Big Mac Combos four days a week.