How do students' poor performance affect teachers?

This answer contains a large range of possibilities.  The consequences for that particular teacher are largely determined by administrators and school districts, so it's possible that anything from zero consequences to firing could happen.  
The real response is likely to be something in the middle of the two.  A teacher that is determined to be doing a poor job of enhancing student learning will likely be placed on some type of probation.  In the beginning, this would probably be the teacher, a master teacher, and an administrator working together to improve the teacher's overall pedagogy.  Student progress would be monitored, and that teacher would likely be observed quite frequently in order to be given feedback on everything from lesson plan set up to classroom management.  Eventually, it's possible that the probation turns into something like a last chance year.  Either the teacher improves and the students show signs of higher learning outcomes, or the teacher is not rehired.  Whether you agree or disagree with judging a teacher's teaching ability based on student progress, it is a somewhat objective way to measure a teacher's effectiveness in a classroom.