If you are writing an original essay or story on the theme of a school trip, it can help to begin with outlining a plot diagram.

Let's start with the climax- this should be the most exciting or intense part of your written piece. So, if your theme is on a school trip, what exciting thing may happen during the trip? The climax may be a very short event, or last the duration of the trip. If the class in your story had been planning a trip to somewhere they were very excited to visit, like a museum, the climax might last for the entire time they are at this location. Alternately, maybe one character in particular is anticipating some sort of conflict which comes to a head during the trip. If your character was afraid of roller coasters and the class took a trip to a theme park, the climax may be how your character confronts their fear of roller coasters.

Once you've decided on the climactic point of your story, it is easier to describe the rising and falling actions. The rising action should be the part of your story where excitement builds. This could include preparations for the school trip, the travel involved in the trip, and anything characters may be anticipating. It's important to consider some obstacles here- what might prevent characters in your story from going on the school trip? 

The falling action is the part of your story after the climax where excitement begins to fall and the story is coming to a close. What happens after the climactic point of your story? The trip home? Maybe your characters learned something during their trip or have some feelings to do with the climax? The falling action should describe how characters react and respond to the climax, and any events which may have been caused by the climax. 

I hope that this has been helpful. I think that once you decide on a setting and climax for your story, it will be easier to plan out the rising and falling actions.