Background There is evidence from around the world that disruptive behaviour during adolescence is associated with increased risk of later criminality. Outcomes for young people placed in the Finnish residential school because of severe conduct problems are not known. Aims Our aims were to investigate criminality after leaving a residential school placement during adolescence, and to compare trends in criminality between four successive graduate cohorts (1991, 1996, 2001 and 2006). Methods We used official records to study complete national cohort of all 861 people who had been resident in the Finnish residential school system on the last day of each of the years 1991, 1996, 2001 and 2006. They were compared with 4255 matched general population controls. The follow‐up time was up to 20 years. Results Two‐thirds (66%) of all residential school graduates (N = 566: 448/78% men, 118/41% women) had at least one criminal conviction in adulthood, a 13‐fold elevation over the general population rate. The most prevalent crime categories were violence (N = 409, 48%: 331/58% men, 78/27% women) and property crimes (N = 405: 47%: 346/60% men, 51/21% women). The risk of violent crime was 18 times that among controls; 13 of residential school males (2.3%) had a homicide conviction. Once adult, the risk of offending decreased with age. There was a significant trend for improvement in offending rates between the earliest and latest residential school cohorts. Conclusions The risk of committing crimes after a residential school placement is sufficiently elevated that alternative strategies, perhaps particularly longer‐term post‐release supervision and aftercare, should be considered. Indications of lower crime rates in later cohorts suggest that some positive changes to the school regime and/or aftercare may have been made already. Copyright © 2017 John Wiley & Sons, Ltd.