By Siri FiskeImagine a company that keeps entry-level workers separate from more senior employees. One team consists exclusively of 22-year-old new hires, another of 45-year-old middle managers, and a third of 60-year-old senior vice presidents. No inter-team collaboration allowed. Such a setup would be a terrible business decision. Upper management would operate in a bubble with zero feedback. And younger employees would suffer from a lack of mentorship. No one would ever structure a company this way. Yet it's exactly how we organize our schools. Segregating students based on age makes little sense. If we instead taught in multi-age classrooms, kids could advance at their own pace, learn from each other, and adapt to the work settings they'll encounter in the real world. Age hasn't always dictated classroom setups. Until the industrial revolution, most classrooms accommodated children of all ages. In fact, teachers often trained older children to instruct younger pupils. These student "monitors" helped a limited number of teachers manage the classroom. But schools ultimately changed to match the industrial revolution's factory-like aesthetic. Much as workers performed different tasks in sectioned-off areas of a factory, students learned age-specific material in age-segregated classrooms. This sort of separation no longer exists in the working world. It's time for schools to change too. Clustering students by age harms pupils who perform well-above or well-below grade level. According to a recent Center for American Progress survey, 36 percent of fourth graders said their math homework was "often" or "almost always" too easy. And 14 percent said their math homework was "never" too easy. Mixed-age classrooms make it easier to group kids according to the skills they've mastered. These classroom setups also offer social advantages. Children in mixed-age classes are more likely to volunteer and less likely to exhibit aggressive behavior. A recent study in the Journal of Youth and Adolescence found that studentswith mixed-grade friendships are less lonely. Kids in mixed-age classrooms thrive academically. "Students from multiage classrooms achieved greater academic outcomes in relation to their abilities and demonstrated greater increases in academic achievement than students of the same and higher abilities from single-age classrooms," according to a research digest from the University of Illinois. It might seem that mixed-age classrooms would hold back high-achieving older students, who have to wait for their younger or less experienced counterparts to learn new material. But in a well-designed classroom, different students or groups of students could be working on different tasks during the same class period. And older students actually benefit from mentoring their younger peers. When they are expected to relay knowledge to others, students work harder to understand and retain information -- a phenomenon known as the "protégé effect." In a recent study in the journal Memory & Cognition, researchers asked one group of participants to learn a passage for a test. They asked another group to learn a passage they'd have to teach to others. Participants who thought they would have to teach the passage recalled the material more easily and performed better when tested. When schools stop letting age dictate their classroom rosters, students surpass expectations. One student entered my school, Mysa School, at what other institutions assumed to be a second-grade- level. Our ability-centered curriculum catapulted that student into third-grade-level math and middle school-level language arts in just six months. Grouping kids by age stifles social and academic potential. It's time for educators to realize that, in the classroom, age really is just a number. Fiske is the founder and head of Mysa School. This piece originally ran in the Houston Chronicle.