What are the most & least stressful grades to teach among K-12, and why?

I'd say early on (K or 1st grade) because for many, this is their first experience being in a classroom. Also, with schools upping the ante on what they want children to have accomplished academically by the end of kindergarten or first grade (math and reading levels have really been boosted up) teachers have a lot of pressure on them. Also, when you consider that what is learned in K and 1, serves as the building block for all learning, it is quite stressful.

I'd also think that teaching from 8th - 12th grade would be extremely stressful. Kids in this age group often have behavioral/respect issues. (Not all, but for those who do, it seems these are the worst years for it). Some have matured by around 11th grade, but others remain just as immature and not committed to learning in 12th grade as they were in 8th. I think the stress in teaching these kids comes from behavior and assessments that the students are expected to pass. A low pass rate for the students of a particular teacher looks really bad and adds to the stress.