One of the main aims of national assessment programmes is to improve the efficacy of education systems; realizing this aim often takes the form of implementing a variety of accountability measures. Using assessment results for accountability purposes is highly controversial, while one of its undesirable impacts is that it generates negative attitudes towards educational assessments among teachers. The aim of this study is to examine lower and upper secondary teachers' (N = 1552) opinions and beliefs about testing and, more specifically, about the national assessment programme in Hungary. A questionnaire was used to explore teachers' beliefs about the effects of the assessment system on how they teach, perceived pressure from stakeholders, teachers' acceptance of assessment programmes and the relationship between these beliefs. Results show that assessment programmes compel teachers to revise their teaching practices -- some change to make meaningful gains in student learning, while others turn to practices that are not conducive to a genuine improvement in students' knowledge, focussing instead on assessment scores. Pressure from inside the school (colleagues and school leaders) and teachers' attitude towards assessments bring about changes in instruction, such as the reallocation of coaching and improvement in teaching. Sources of pressure outside school (local government and the media) have an indirect effect on changes in teaching because their pressure influences in-school motivators. Pressure from parents and students is felt directly by teachers, but only in limited areas. The results demonstrate that a national assessment programme has a more significant impact on teaching in lower than in upper secondary schools.