If you teach in an all-white, middle-class district, it's easy to have decent test scores without much real instruction happening. The students and their literacy-minded parents (all those years of bedtime stories in English! All those narrated trips to the zoo!) are what makes the school score well--the teachers may well be engaging in touchy-feely instructional methods that don't do anything for real academic growth (like, say, endless student presentations--I see this in white schools all the time). 
When a school is specifically responsible for teaching students who don't come in with the skills they need--you see of that school has real instructional chops. 
Teachers unions, of course, resent the idea that teachers should be accountable for student learning and will make a hundred excuses about why poor children "can't" learn.