Schools with primarily indigenous and ethnic minorities in low socioeconomic areas have long been associated with low levels of achievement, particularly in literacy. This is true for New Zealand despite high levels of reading comprehension by international comparisons (e.g., PISA). Recent reviews of schooling improvement suggest small gains over the short term are possible with well-designed interventions, but for children in the middle primary school years, the criterion against which effective interventions need to be judged is sustained and systematic acceleration across levels of achievement in order to achieve equitable distributions of achievement. Plotting gains across time is also needed to examine whether "summer effects" can be overcome. The present quasi-experimental design study was a three-year research and development collaboration among schools, government, and researchers to raise reading comprehension through critical discussions of achievement and teacher observation data and linking research on effective comprehension practices to specific needs. The collaboration resulted in increased rates of achievement that were variable but sustained across three years. The growth model showed a step-like pattern with rapid gains over school months and a plateau over summer. Over three years this represented an average achievement gain across cohorts followed longitudinally of one year's progress in addition to expected progress over that period with stanine effect sizes of d = 0.62. The results show the significance of testing effects against the criterion of sustained and systematic achievement and the need to examine growth over multiple calendar years to better represent the pattern of gains. (Contains 6 figures, 7 tables, and 1 note.)