This was surprising too:
We should also keep in mind that, even in the best schools, many children do not succeed in
achieving their growth index target. In the top 10% of schools about 60 to 70% of the students
reach their RIT point growth norm target in reading, while about 65 to 75% of students meet this
target in mathematics. When setting targets using this statistic, educators should remember that a
school in which more than 70% of the students meet or exceed their growth index norm is
exemplary.
http://illinoisnwea.wikispaces.com/file/view/SchoolGrowthNormStudy2006-Final.040407.pdf
