The Education Writers Association has produced a brief on teacher effectiveness, which you can read in full here.

it’s important for journalists and others to understand what is known about the topic so far, and what remains unsettled or unknown. This research brief does not synthesize all the studies in this highly technical field. But it does aim to improve the accuracy and clarity of reporting by exploring what the research says about timely questions surrounding the complex topic of teacher effectiveness.

The brief is organized around several prevailing questions about teacher effectiveness in K-12 education. For each question, we’ve reviewed some of the most-important research, identifying key findings and tension points. (Citations in the text refer to a list of sources in the bibliography.) At the end of each section, we present a bottom-line summary of the research.

Nearly all of the studies cited here rely on the use of student test scores as a proxy for learning, a research practice that remains hotly debated. A full discussion of the value of standardized testing lies outside the scope of this paper, but we begin from the same assumption as many scholars: that standardized tests measure important aspects of student learning, but not the full breadth and depth of what students should know and be able to do.

The brief draws on a review of over 40 specific research studies or research syntheses, as well as interviews with scholars who have used primarily quantitative research methods to analyze the relationships between teachers, their attributes, and student achievement.

Here's a summary of the brief

Are teachers the most important factor affecting student achievement?

It can be said:
Research has shown that the variation in student achievement is predominantly a product of individual and family background characteristics. Of the school factors that have been isolated for study, teachers are probably the most important determinants of how students will perform on standardized tests.

Are value-added estimations reliable or stable?

It can be said:
Value-added models appear to pick up some differences in teacher quality, but they can be influenced by a number of factors, such as the statistical controls selected. They may also be affected by the characteristics of schools and peers. The impact of unmeasured factors in schools, such as principals and choice of curriculum, is less clear.

Does merit pay for teachers produce better student achievement or retain more-effective teachers?

It can be said:
In the United States, merit pay exclusively focused on rewarding teachers whose students produce gains has not been shown to improve student achievement, though some international studies show positive effects. Research has been mixed on comprehensive pay models that incorporate other elements, such as professional development. Scholars are still examining whether such programs might work over time by attracting more effective teachers.

Do students in unionized states do better than students in states without unions?

It can be said:
Students tend to do well in some heavily unionized states, but it isn’t possible to conclude that it is the presence or absence of unions that cause that achievement.