It is increasingly popular for primary schools to run 'learning themes' within classes. Often a whole year group, keystage or even the whole school will be on-board. The idea is to put learning into a context which engages children's imaginations and encourages them to make creative-leaps in their thinking. Older teachers know this as 'topic' work!
If you are aware of the class topic or learning theme before you arrive you can prepare yourself with activities which link in with it. You can benefit from the classes pre-developed interest. Knowing what the teacher has planned is useful so that you don't recover the same ground, but it is not overly important. The likely-hood of treading on toes isn't high. However, it might be a good idea to choose an activity which avoids the usual theme clichés.
Even if you struggle to make a relevant activity fit into a theme, remember that children love stories. I find that throwing in 'factoids' can help make links to the topic - however tenuous!
E.g. Class 6 is studying 'Victorians', but have been asked to practise averages in maths. Try being expressive and creative telling a story about how injuries were common in Victorian cotton-mills, especially amongst children as they ran beneath the mechanical looms. Then generate data that might reflect injury rates.
E.g. Class 3's learning theme is 'rainforests'. They are doing science and are looking at different materials to investigate their properties. Make a link to the theme by suggesting that they look for a material that would be just right for building a tent out of. Not too heavy, thick, etc, and definitely waterproof.
Tip: have the children produce their learning on loose plain paper. This gives their class teacher the option of building a display from it to add to the class theme.