In an attempt to accelerate the development of formal operations in average young adolescents, intervention lessons relating to all formal schemata were designed in the context of school science courses. Over a period of two years, up to 30 intervention lessons were given by science teachers to their classes in eight schools. Boys who started the program aged 12+ showed a pre-posttest effect size on Piagetian tests of 0.89 SD compared with control classes. In terms of British norms for the development of operational thinking this was a mean change from the 51st to the 74th percentile. Neither the middle school students nor the 12+ girls showed greater gain than the controls. Gains were shown by girls in one 11+ class and in the two 11+ laboratory classes. In the laboratory school students given intervention lessons by the researchers maintained their gains over controls in formal operations at a delayed posttest one year after cessation of the program. There was no effect on tests of science achievement during the intervention. It was argued that the interventions needed to be accompanied by in-service training designed to enable teachers to change their teaching style in line with their students' increased operational thinking capacity.