Can the quality of students' work be improved through training in self- assessment practices? This paper considers the impact of training high school students on their performance in external examinations. Teachers were selected from a sample of high schools and trained in how to develop students' self-assessment skills. Ten high schools representative of the top, middle and bottom levels of academic achievement in national examinations were chosen and students trained in self-assessment by their normal class teachers as part of their final year curriculum. An experimental group comprising 256 participants received formal training in self-assessment skills for the entire three terms of the academic year. A control group was selected from matched classes not receiving such training. A significant difference favouring those trained in self-assessment was found overall and in each curriculum area. While it is demonstrated that self-assessment training can have an impact on student performance the paper considers the circumstances of the study and whether similar outcomes might be possible in less favourable conditions.