How many hours do teachers work?

Teachers "officially" work 35-40 hours a week

That's actually at the school.

Teachers are expected to be present at school each day, which means that they have to arrive before the students and they leave after students leave. Add on the time spent marking and grading papers at home; preparing report cards; shopping for supplies (average teacher spends $250 - $500 a year out of his/her own pocket for classroom supplies); getting materials together and preparing worksheets; private research and reading; professional development requirements and continuing education classes; "meet-the-teacher" night and other parent-teacher interviews; staff meetings; extra-curriculum activities such as coaching sports or music groups and entering competitions for same; and you can easily figure an average of 50 hours a week, including weekends.