This study examined the development of social skills across five measurement points from 4th through 7th grade, and the influence of child gender and school-related factors on the level and growth of social skills, in a large sample of normally developing children in Norway (N = 2,076). On average, children's social skills scores increased slightly, girls received higher scores than boys, and individual order stability was fairly high over time. Growth mixture modeling (GMM) identified three distinct trajectory classes, one with stable average scores over time (72%), and two (14% each) with high initial and declining scores and low initial and increasing scores, respectively. The school-related factors (e.g., student-teacher relationships) predicted social skills development differently within the trajectory classes.