Here is a report on working hours of teachers.

http://www.bls.gov/opub/mlr/2008/03/art4full.pdf

Teachers work 24 fewer minutes per weekday and 42 minutes per Saturday than other professionals. They worked a little bit more on Sundays, I'd guesstimate about 10 minutes based on the graph. (The weekday and saturday numbers are given in the text, the sunday number is not.)

Table 5 suggests this works out to about 7 hours and 15 minutes per weekday, and about 1 hour per saturday, maybe 1.5 hours per sunday. That's just shy of 39 hours/week, for 10 months/year.