The main purpose of this study was to examine the classroom, teacher and student factors distinguishing grade three classes performing at higher levels than expected, in relation to socioeconomic status (SES) and language factors, from classes performing below their potential with regard to the same factors. Data from a standardized reading comprehension test and student and teacher questionnaires covering teacher, classroom and student characteristics were collected. The participants were 1,092 grade three classes and their class teachers, from Stockholm, Sweden. By use of regression and a twin-matching procedure, one group of 94 underachieving classes and another group of 94 overachieving classes were formed for comparison. Data about extended voluntary reading, classroom climate, teacher experience and the use of authentic literature were seen to be the main four indicators discriminating between over- and underachieving classes beyond the impact of SES and language background. (Contains 7 tables.)