Across the nation, educators are reviving an old idea: separate the girls from the boys?and at Roncalli Middle School, in Pueblo, Colo., administrators say, it's helping kids of both genders. This past fall, with the blessing of parents, school guidance counselor Mike Horton assigned a random group of 50 sixth graders to single-sex classes in core subjects. These days, when sixth-grade science teacher Pat Farrell assigns an earth-science lab on measuring crystals, the girls collect their materials?a Bunsen burner, a beaker of phenyl salicylate and a spoon. Then they read the directions and follow the sequence from beginning to end. The first things boys do is ask, "Can we eat this?" They're less organized, Farrell notes, but sometimes, "they're willing to go beyond what the lab asks them to do." With this in mind, he hands out written instructions to both classes but now goes over them step by step for the boys. Although it's too soon to declare victory, there are some positive signs: the shyest boys are participating more. This fall, the all-girl class did best in math, English and science, followed by the all-boy class and then coed classes.