If you are a teacher in a primary school then the chances are you will have to plan class assemblies from time to time. Trying to coordinate an assembly can be quite stressful and the key is to give yourself plenty of time. Firstly you need to work out what you will be focusing on in the assembly. It may be that the children have been doing work on a topic that you want to talk about or their experience on a recent school trip from example.
Have a look online for ideas that you may be able to use and materials that may be useful. Once you have an idea start to think which children will have which role. It is best to give the most confident children the roles that involve a lot of interaction or speaking but be sure to give everyone the chance to say something. It is often nice to get the children to whole up a piece of work they have completed or read it out if it is a poem for example.
For younger children, often including a song can make it fun and enjoyable and will help the children who find it hard to stand up in front of others, have more confidence.