Substitute teachers study

Growing use of substitutes may hurt scores
Across the country, twice as many schools reported using substitutes in 2004 to fill long-term teaching vacancies versus a decade earlier, according to U.S. Education Department data. New research suggests that teacher absences lead to lower student test scores, even when substitutes are brought in.
National Center for Education Statistics. (2007). Education Department Schools and Staffing Survey. Washington, DC: NCES.
http://nces.ed.gov/surveys/sass/