Part of the problem with doing that, especially in younger grades, is that so much of it depends on variables outside of a teacher's control. A lot of reading at a young age comes from life experiences. For instance, someone who has never been outside of an inner city is going to have more trouble reading a story about someone sailing on the ocean because they are so far removed from even the most basic plot elements that the things in the story that are actually meant to challenge are not ever gotten to. Additionally, time spent outside of classroom compounds the effectiveness of time spent inside of classroom, but the teacher has no control over that. A teacher can't even control if a student shows up for school or not. Student's performance, especially cross-school comparisons, are nearly worthless and would unfairly punish those that teach at low schools.
I would say you'd have to compare them to similar schools in the area or similar areas in the U.S. Between income range in the surround neighborhoods and population density you should be able to come up with a decent comparison, I would think.
