ok, so maybe the teachers you know don't work during the summer (although I wonder if this would be true if you really talked to them about what they do). Regardless, most teachers DO work quite a bit over the summer.

http://www.usnews.com/opinion/knowledge-bank/2015/07/15/school-is-out-but-teachers-are-still-working

They often are teaching, doing training, preparing curriculum, doing work developing state curriculum, etc.

There are other articles/data you could look at too.