It depends on what sort of school you're talking about the number of students at the school.
U.S. Department of Education National Center for Education Statistics says the average national student to teacher ratio is 15.38 students to every teacher.
If you need the source for a citation, the source is the U.S. Department of Education, National Center for Education Statistics, Common Core of Data (CCD), "State Nonfiscal Public Elementary/Secondary Education Survey", 2009-10, v.1b.
You can find this information here:
http://nces.ed.gov/ccd/elsi/quickFacts.aspx