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Yesterday, a California Judge ruled in Vergara that keeping ineffective teachers in schools, particularly when these teachers are assigned to minority populations, violated the students civil rights to equality of educational opportunity.  This ruling was predicated on how damaging having ineffective teachers is to children. There are several striking quotes in the California-Decision that prove how important teacher effectiveness is.

All sides to this litigation agree that competent teachers are a
critical, if not the most important, component of success of a child’s in-
school educational experience. All sides also agree that grossly ineffective
teachers substantially undermine the ability of that child to succeed in

Evidence has been elicited in this trial of the specific effect of
grossly ineffective teachers on students. The evidence is compelling.
Indeed, it shocks the conscience. Based on a massive study, Dr. Chetty
testified that a single year in a classroom with a grossly ineffective
teacher costs students $1.4 million in lifetime earnings per classroom.
Based on a 4 year study, Dr. Kane testified that students in LAUSD who are
taught by a teacher in the bottom 5% of competence lose 9. 54 months of
learning in a single year compared to students with average teachers.

Wow, a student with a teacher in the bottom 5%  loses 9.54 months of learning compared to a teacher at the 50th Percentile! Since most school years are 9 months, this is a loss of a year of learning.