This shows the number of students per full-time equivalent (FTE) teacher. A low student-teacher ratio is not the same as a small class size. Some schools have teachers who teach specialized subjects with smaller classes. Other schools hire extra teachers to free up class preparation time for their core teaching staff. Both of these possibilities lower the overall student-teacher ratio but not the average class size.
Note that student-teacher ratios based upon FTE teachers can be exceedingly high for schools that have a large number of part-time teachers. If, for example, a small school has four part-time teachers at 25%, the student-teacher ratio at this school would be calculated based upon one teacher instead of four (because four teachers at 25% is equal to one full-time teacher). This in turn will make the student-teacher ratio appear higher than it really is. If your school has an exceptionally high student-teacher ratio, check with your school principal for an explanation.
Source: The school level student-teacher ratio is from the VA Dept. of Education for the 2007-2008 school year. The state average was calculated by GreatSchools.net using the total number of students enrolled and the total number of FTE teachers provided by the VA Dept. of Education for 2007-2008.