The American Statistical Association (ASA) the nationas leading group of statisticians recently released a statement of Valuee-add, that calls into question the whole premise of using this statistical measure for the purposes of evaluating teachers job performance. Their comments below:

VAMs are complex statistical models, and high-level statistical expertise is needed to
develop the models and [emphasis added] interpret their results.
Estimates from VAMs should always be accompanied by measures of precision and a discussion of the assumptions and possible limitations of the model. These limitations are particularly relevant if VAMs are used for high-stakes purposes.
VAMs are generally based on standardized test scores, and do not directly measure
potential teacher contributions toward other student outcomes.
VAMs typically measure correlation, not causation: Effects – positive or negative –
attributed to a teacher may actually be caused by other factors that are not captured in the model.
Under some conditions, VAM scores and rankings can change substantially when a
different model or test is used, and a thorough analysis should be undertaken to
evaluate the sensitivity of estimates to different models.
VAMs should be viewed within the context of quality improvement, which distinguishes aspects of quality that can be attributed to the system from those that can be attributed to individual teachers, teacher preparation programs, or schools.
Most VAM studies find that teachers account for about 1% to 14% of the variability in test scores, and that the majority of opportunities for quality improvement are found in the system-level conditions. Ranking teachers by their VAM scores can have unintended consequences that reduce quality.
Attaching too much importance to a single item of quantitative information is counter-productive—in fact, it can be detrimental to the goal of improving quality.
When used appropriately, VAMs may provide quantitative information that is relevant for improving education processes…[but only if used for descriptive/description purposes]. Otherwise, using VAM scores to improve education requires that they provide meaningful information about a teacher’s ability to promote student learning…[and they just do not do this at this point, as there is no research evidence to support this ideal].
A decision to use VAMs for teacher evaluations might change the way the tests are viewed and lead to changes in the school environment. For example, more classroom time might be spent on test preparation and on specific content from the test at the exclusion of content that may lead to better long-term learning gains or motivation for students. Certain schools may be hard to staff if there is a perception that it is harder for teachers to achieve good VAM scores when working in them. Overreliance on VAM scores may foster a competitive environment, discouraging collaboration and efforts to improve the educational system as a whole.

You can read their whole document below.

ASA Statement on Using Value-Added Models for Educational Assessment