There are several significant problems here, regardless of how the study turns out. Is two years really enough?
This NYT article describes the difficulties in rating teachers:
But the experience in New York City shows just how difficult it can be to come up with a system that gains acceptance as being fair and accurate. The rankings are based on an algorithm that few other than statisticians can understand, and on tests that the state has said were too narrow and predictable. Most teachers’ scores fall somewhere in a wide range, with perfection statistically impossible. And the system has also suffered from the everyday problems inherent in managing busy urban schools, like the challenge of using old files and computer databases to ensure that the right teachers are matched to the right students.
Has Texas found some better way to rate teachers? If so, they should give New York a call, but unfortunately it seems that Texas is only looking at two years of "student achievement levels." That means test scores. According to a study mentioned in the same NYT article, "with one year of data, a teacher was likely to be misclassified 35 percent of the time. With three years of data, the error rate was 25 percent. With 10 years of data, the error rate dropped to 12 percent." Again, Texas apparently has two years of data - but, to be fair, how could they have more when TFAers only commit to teach for two years?
So let's put all that aside and say that a TFA teacher can raise test scores for a year or two. Does that make them a better teacher? Or does that mean that they are better at teaching to the test? It may feel good to base decisions on numbers, but there's just no way to be sure. Then there's the long-term impact on schools that have teachers turning over every two years on a regular basis. How do you establish a community? Do parents really want to think of their child's teachers as long-term temps?
Texas is spending $8,000 over two years on top of salaries for each of their TFA teachers, and yet they have to commission a study to figure out if the teachers are worth it? I'd like to know what exactly they expected to get for their $4,000 per year per teacher that was above and beyond what their regular, credentialed teachers are doing.