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Large scale reduction in class sizes can have unintended consequences. The experience of California is the best-known example. To reduce K-3 class sizes from 30 to 20, California had to fill 25,000 new teaching positions in two years. The teachers hired were inexperienced and many were uncertified (Jepsen & Rivkin 2009). When teaching vacancies were created, many senior teachers took the opportunity to transfer to schools serving more affluent white and affluent Asian students, and as a result the inexperienced and uncertified so that the new, inexperienced teachers were disproportionately hired by schools serving poor black and Hispanic students (Jepsen & Rivkin 2009). In addition, many schools lacked enough classrooms for the new teachers, and had to use portable classrooms or staggered year-round calendars to accommodate them. The year-round calendars slightly reduced test scores, in part by making it harder to attract and retain experienced teachers in high-poverty schools (von Hippel 2016; and citations therein).
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