Backstory is always a good route. If you want to flesh out your characters more expand on some of the not school/destiny related things they enjoy. Maybe they go on a holiday from school and they can see their families again? Maybe they explore the school more?
Maybe take the time to flesh out the type of society they live in. Are there other places in space they can go to beside the school? What is earth like and do people come from different planets? Are there any aliens around here? It can help to ask questions and answer them, seeing if you have already put that into the story.
Maybe the main characters go off exploring away from the school, or inside if its an especially fantastic Academy of Adventure
, and they meet some alien creature a befriend him. He could flesh out the supporting cast as well as giving you something interesting to talk about. Alternatively, maybe there are other students or teachers around that you want to focus on and give some characterization.The thing about filler is it can't seem like your trying to pad stuff about. The best thing to do to add more material to the story is to fill out the cast and world build. It's usually better than adding some random arch that goes nowhere about going to a space beach
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