"Irreplaceables" are teachers who are so successful they are nearly impossible to replace, but who too often vanish from schools as the result of neglect and inattention.To identify and better understand the experience of these teachers, we started by studying 90,000 teachers across four large, geographically diverse urban school districts.

We also examined student academic growth data or value-added results for approximately 20,000 of those teachers. While these measures cannot provide a complete picture of a teacher's performance or ability on their own -- and shouldn't be the only measure used in realworld teacher evaluations -- they are the most practical way to identify trends in a study of this scale, and research has demonstrated that they show a relationship to other performance measures, such as classroom observations.

We used the data to identify teachers who performed exceptionally well (by helping students make much more academic progress than expected), and to see how their experiences and opinions about their work differed from other teachers' -- particularly teachers whose performance was exceptionally poor.

So who are the Irreplaceables? They are, by any measure, our very best teachers. Across the districts we studied, about 20 percent of teachers fell into the category. On average, each year they help students learn two to three additional months' worth of math and reading compared with the average teacher, and five to six months more compared to low-performing teachers.

Better test scores are just the beginning: Students whose teachers help them make these kinds of gains are more likely to go to college and earn higher salaries as adults, and they are less likely to become teenage parents.Teachers of this caliber not only get outstanding academic results, but also provide a more engaging learning experience for students. For example, when placed in the classroom of an Irreplaceable secondary math teacher, students are much more likely to say that their teacher cares, does not let them give up when things get difficult and makes learning enjoyable.

Irreplaceables influence students for life, and their talents make them invaluable assets to their schools. The problem is, their schools don't seem to know it.