1,097: Average number of hours U.S. teachers spend per year on instruction.

Students across the U.S. are enjoying or getting ready for summer vacation, but teachers may be looking forward to the break even more. American teachers are the most productive among major developed countries, according to Organization for Economic Cooperation and Development data from 2008 -- the most recent available.

Among 27 member nations tracked by the OECD, U.S. primary-school educators spent 1,097 hours a year teaching despite only spending 36 weeks a year in the classroom -- among the lowest among the countries tracked. That was more than 100 hours more than New Zealand, in second place at 985 hours, despite students in that country going to school for 39 weeks. The OECD average is 786 hours.

And that's just the time teachers spend on instruction. Including hours teachers spend on work at home and outside the classroom, American primary-school educators spend 1,913 working in a year. According to data from the comparable year in a Labor Department survey, an average full-time employee works 1,932 hours a year spread out over 48 weeks (excluding two weeks vacation and federal holidays).

Despite the amount of time that teachers spend working, student achievement in the U.S. remains average in reading and science and slightly below average in math when compared to other nations in a separate OECD report. That remains a concern as education is one of the most important ways a country can foster long-term economic growth.