It sounds like you're not the type to let nonsense like the "Teacher Report Cards" get you down, which is good. You probably don't even need the words of encouragement I want to offer you; it seems like your supervisor and your students have all the words of encouragement you could ever want.
From those who know you best, the children and adults with whom you work every day, we hear nothing but praise. If a mathematical formula says otherwise, well, frankly, screw it. As Albert Einstein wisely observed, "Not everything that counts can be counted, and not everything that can be counted counts."
And in your sad case study, we see the (perhaps unintended) results of the value-added craze: A teacher who is clearly dedicated to her students, who are quite obviously high-achieving children, is somehow determined to be deeply inadequate because her already high-achieving children fall a few hundredths of a point below where they "should" be. Never mind the fact that this shortfall has not prevented her students from gaining admission to the city's most exclusive high schools. No, Ms. Isaacson, a fine teacher by any other commonsensical measure, is deemed to be far below average.
How many Ms. Isaacsons will be lost in the obsession with "data"? I hope that not even one will be lost--especially not you, Ms. Isaacson, yourself.