Class size refers to the number of students a teacher faces during a given period of instruction.
Measurements and definitions
Some researchers and policymakers have studied the effects of class size by using student-teacher ratio (or its inverse, teacher-student ratio), but class size is not accurately captured by this metric. As Michael Boozer and Cecilia Rouse explain in "Intraschool Variation in Class Size: Patterns and Implications", student-teacher ratio gives an imprecise view of class size because teachers may be unevenly distributed across classrooms. Some teachers have light course loads as they are assigned to spend most or all of their time coaching other teachers. These coaches would nevertheless factor into the calculation of student-teacher ratio. In other classes – say, an inclusion class with special education students – two teachers may jointly teach a class of thirty-four students. Although student-teacher ratio would describe this class' size as seventeen, these teachers continue to face thirty-four students during instruction. In general, average class size will be larger than student-teacher ratio anytime a school assigns more than one teacher to some classrooms. In poor and urban districts, where schools enroll higher numbers of students needing specialized instruction, student-teacher ratios will therefore be especially imprecise measures of class size.
Although student-teacher ratio does not measure class size, some important studies and surveys have used student-teacher ratio as a proxy for class size. Indeed, some critics of class size reduction, including Malcolm Gladwell's David and Goliath, cite a 1986 study by Eric Hanushek, "The Economics of Schooling: Production and Efficiency in Public Schools", that relies on a literature review of data on student-teacher ratio instead of class size. See Class-size reduction for a full discussion of Hanushek's thesis.
Class size through history
Educators have noted the benefits of class size since classical times.
Isocrates opened an academy of rhetoric in Athens around 392 B.C.E to train Athenian generals and statesmen, and he insisted on enrolling no more than six or eight students in his school at a time. Edward J. Power explains that Isocrates admitted "only a few students to his classes because of his extraordinary concern for care." Quintilian, a rhetorician writing in the Roman Empire around 100 CE, cited the practices in Isocrates' school as evidence that a caring education required small class sizes. Quintilian argued in Institutes of Oratory, as Edward Power summarizes the book's thesis, that "care had nothing whatever to do with discipline: it meant simply that only a few students at a time could be taught effectively."
The twelfth century rabbinic scholar Maimonides recognized that class size was correlated with student achievement. He wrote: "Twenty-five children may be put in charge of one teacher. If the number in the class exceeds twenty- five but is not more than forty, he should have an assistant to help with the instruction. If there are more than forty, two teachers must be appointed.''
Erasmus, the Dutch Humanist, wrote in his 1529 study of education De Pueris Instituendis about the advantages of private tutoring over ecclesiastic and public schools, where he believed classes had grown too large. He explained that "his standard of efficiency demanded a small school conducted by brilliant scholars…" Erasmus recognized that most parents would nevertheless have to settle for large class sizes because of the financial costs of such tutoring.
At the turn of the 20th century, the philosopher and educational theorist John Dewey explained that in his ideal school, class sizes should be very small. "For the purposes of convenience, the children are divided into small groups of eight to twelve according to the kind of work and the age of the children. It is expected that the teacher will give attention to the specific powers and deficiencies of each child, so that the individual capacities will be brought out, and individual limitations made good."