This study investigated the effectiveness of guidance and counselling services on secondary school students' behavioural change. A sample of 100 respondents including teachers and school counsellors was drawn from ten public secondary schools in Nyamagana district, Mwanza Region. The research method used was survey; data were obtained through questionnaires with closed and open - ended questions. Data was analysed and presented in the form of pie charts and histograms by using excel package. The findings from the study indicated that, the presence of school counsellors at any school is very important, because guidance and counselling services were proved to be crucial factor in students' behavioural change. Based on the findings of this study, the researcher recommends that parents, teachers and school counsellors should take more time to counsel and guide students in order to help them change their behaviour.