The topics of school assembly speeches are typically decided by the speaker in conjunction with the school administration. While there are no formal rules or processes dictating the best possible topic for a school assembly speech, topics typically include behavioural issues, school identity, personal development, safety and developmental issues.
Because school assemblies typically involve the entire student population congregated into a singular space, they often involve administrative issues such as policy changes and organizational issues. School assembly speeches are also ideal opportunities to promote school events and extracurricular activities that the school administration wants students to participate in. School assemblies also tend to be the arena for principles to address serious disciplinary issues with the entire population.
School assembly talks certainly are not limited to these administrative topics. Many school assemblies feature speakers from the community, in which case those speakers are encouraged to speak on their area of expertise as it relates to the student population. Talks like this encourage the formation and education of students in a real world setting. Policemen may talk about the importance of safe driving and drug abstinence, while a doctor may talk about the necessity of education as a means of eventually procuring a lucrative job. These sorts of talks serve as a break from the monotonous routines of a regular class day.