Classrooms are becoming more diverse and the number of low achieving students is continuously increasing. About 20% of European 15-year-old-children are not at the required educational levels. There are many reasons for students’ low achievement such as negative attitudes, immigrant or low socio-economic backgrounds, learning difficulties etc. Furthermore, students find that science and especially chemistry is boring, uninteresting, irrelevant and difficult. Nevertheless, low achievement can have critical consequences for the entire society, if students are unable to participate in it as equal citizens, make rational and sustainable decisions or be sufficiently qualified for working life demands. Accordingly, more research is needed, how low achieving students could be supported in diverse classrooms. Science teachers tend have more negative beliefs towards student diversity and teaching practices in diverse classrooms compared to subject teachers. That is crucial, because teachers' beliefs and practices can affect students' achievement. The aim of this thesis is to understand, how science, and especially chemistry teachers could be better supported and prepared for diversity and better student achievement during their entire careers. The main research question is: What is the relationship between student achievement and teachers' beliefs about diversity and science teaching practices for diverse students? In order to answer to the main research question, three subquestions are asked. Firstly, low achieving students' thoughts about chemistry and chemistry teaching practices compared to other students is analyzed. The second question clarifies, how does the STSE-based school-industry collaboration affect to pre-service science teachers' beliefs about their future practices. The third question is about pre- and in-service teachers' beliefs about teaching chemistry in diverse classrooms. This thesis consists of three interconnected studies and one descriptive report (I-IV). Study I describe what is the connection between students' low achievement and most preferred teaching practices compared to other students. Since there was a clear evidence of the fact, that all students in spite of their achievement levels preferred industry visits, it was chosen to be a main topic in study II. In that study, the connection between pre-service teachers' beliefs and STSE-based teaching practices were examined in a context of school-company collaboration and visits. Since pre-service teachers' beliefs were significantly improved after the STSE-based course, the following descriptive report III concentrated on science teacher education and how it could be developed in order to prepare future chemistry teachers for diversity. Furthermore, the last study IV is about the in-service chemistry teachers' beliefs about their work in diverse classrooms. The mixed methods approach which includes survey and case-study is used in order to answer the research questions. Data for the survey was collected from 2949 secondary school students with the help of the Finnish National Board of Education (study I). Secondly, the effects of STSE-based teaching practices were carried out in Finland and Slovenia with 42 pre-service chemistry and science teachers (study II). The presented teaching model for teaching diverse students (report III) was based on German and Finnish chemistry teacher education programs. Furthermore, the beliefs of eight in-service science teachers were examined by case-study (study IV). There is not much national or international research about the topic of this thesis. This thesis presents prominent insights and ideas, how especially low achieving students could be supported by developing science and chemistry teacher education. Accordingly, following suggestions are made: (i) Practices for teaching diverse students have an influence on students' achievement. Therefore, it is beneficial to take students' thoughts into account when activities are planned and implemented. It is worth noticing, that students prefer similar practices in spite of their achievement levels: company visits, using digital implementations and working in groups. (ii) STSE-based teaching practices such as company visits are preferred by diverse students and they also have a positive effect on teachers' beliefs. For that reason, teachers should have regular opportunities to practice those skills in authentic environments. STSE-based teaching material can be also beneficial for diverse students. (iii) In-service teachers do have basic knowledge about diversity, and they use a considerable amount of effort in order to take their students' needs into account. However, they need more support and resources. In conclusion, pre-and in-service teachers' concerns, needs and beliefs should be taken into account, when support for them is planned during their entire career. Teacher support for diversity and better student achievement need both national and international collaboration among teacher educators, pre- and in-service teachers, special education teachers, students, parents and other community members. This has a significant effect on student achievement and the entire society in turn.