Being a teacher is the easy life. Just about every other job in the universe is a worse job with worse pay. Your day ends at 4pm, you get 3-4 months off a year and your paid more than well enough to make a living, especially for the average teacher's time spent. They should just fire all of them every 5 years and only give the ones that haven't gone insane yet their jobs back after some very stringent tests.
My mother got chewed out by the union once because she wanted the faculty of teachers she worked with to do the standardized grade 10 literacy test so they could better prepare the students to take them. The test was given, filled out, and graded; not for any sort of evaluation purposes, just for self education. When the union found out that any sort of test with a score, regardless of the purpose behind taking it, was given to the teachers she nearly got fired.
The reason, in all likelihood that teachers' unions are so incensed about being evaluated is because 15% of their members aren't likely able to answer questions about material they're teaching, which is usually still a far cry from what the students actually have to learn. The education systems in the USA and all of North America, for that matter, are nothing more than a glorified babysitting service. Don't put your babysitters under any sort of scrutiny though, or they'll go on strike.