Students' development matters for teachers, but the way it matters depends partly on how schooling is organized. In teaching a single, "self-contained" grade-level, the benefits of knowing about development will be less explicit, but just as real, as if you teach many grade levels. Working exclusively with a single grade (like, say, a third- grade classroom) highlights differences among students that happen in spite of their similar ages, and obscures similarities that happen because of having similar ages. Under these conditions it is still easy to notice students' diversity, but harder to know how much of it comes from differences in long-term development, compared to differences in short-term experiences. Knowledge about long term changes is still useful, however, in planning appropriate activities and in holding appropriate expectations about students. What changes in students can you expect relatively soon simply from your current program of activities, and which ones may take a year or more to show up? This is a question that developmental psychology can help to answer.
If you teach multiple grade levels, as often is true of specialists or teachers in middle school or high school, then your need for developmental knowledge will be more obvious because you will confront wide age differences on a daily basis. As a physical education teacher, for example, you may teach kindergarten children at one time during the day, but sixth-graders at another time, or teach seventh-graders at one time but twelfth-graders at another. Students will differ more obviously because of age, in addition to differing because of other factors like their skills or knowledge learned recently. Nonetheless, the instructional challenge will be the same as the one faced by teachers of single-grade classes: you will want to know what activities and expectations are appropriate for your students. To answer this question, you will need to know something not only about how your students are unique, but also about general trends of development during childhood and adolescence.
Note that developmental trends vary in two important ways. The first, as indicated already, is in their generality. Some theories or models of development boldly assert that certain changes happen to virtually every person on the planet, and often at relatively predictable points in life. For example, a theory might assert that virtually every toddler acquires a spoken language, or that every teenager forms a sense of personal identity. Individuals who do not experience these developments would be rare, though not necessarily disabled as a result. Other theories propose developmental changes that are more limited, claiming only that the changes happen to some people or only under certain conditions. Developing a female gender role, for example, does not happen to everyone, but only to the females in a population, and the details vary according to the family, community, or society in which a child lives.
The second way that developmental trends vary is in how strictly they are sequenced and hierarchical. In some views of development, changes are thought to happen in a specific order and to build on each other— sort of a "staircase" model of development (Case, 1991, 1996). For example, a developmental psychologist (and many of the rest of us) might argue that young people must have tangible, hands-on experience with new materials before they can reason about the materials in the abstract. The order cannot be reversed. In other views of development, change happens, but not with a sequence or end point that is uniform. This sort of change is more like a "kaleidoscope" than a staircase (Levinson, 1990; Lewis, 1997; Harris, 2006). A person who becomes permanently disabled, for example, may experience complex long-term changes in personal values and priorities that are different both in timing and content from most people's developmental pathway.
In general, educational psychologists have tended to emphasize explanations of development that are relatively general, universal and sequential, rather than specific to particular cultures or that are unsequenced and kaleidoscopic (see, for example, Woolfolk, 2006, Chapter 3; or Slavin, 2005, Chapters 8 and 9). Such models (sometimes called "grand theories") have the advantage of concisely integrating many features of development, while also describing the kind of people children or adolescents usually end up to be. The preference for integrative perspectives makes sense given educators' need to work with and teach large numbers of diverse students both efficiently and effectively. But the approach also risks overgeneralizing or oversimplifying the experiences of particular children and youth. It can also confuse what does happen as certain children (like the middle-class ones) develop with what should happen to children. To understand this point, imagine two children of about the same age who have dramatically very different childhood experiences— for example, one who grows up in poverty and another who grows up financially well-off. In what sense can we say that these two children experience the same underlying developmental changes as they grow up? And how much should they even be expected to do so? Developmental psychology, and especially the broad theories of developmental psychology, highlight the "sameness" or common ground between these two children. As such, it serves as counterpoint to knowledge of their obvious uniqueness, and places their uniqueness in broader perspective.