Either you can clean it yourself or you can ask the janitor to clean it. If the janitor doesn't you are left with the option of reporting the matter to the principal. To make your school a cleaner place you can suggest or encourage your peers to not litter in the hallway or school grounds as it will only make your environment unhealthy and chaotic. Who doesn't want to live in a clean , organised, healthy , disease free environment . To encourage people to omit littering you need to make them aware of the benefits of not doing it and the disadvantages to the health and image of the school.. Installing more garbage bins is also a good options for which you can approach your principal for funds. Fining the wrongdoers wont be a bad option in deterring  them from littering.These are just  some random suggestions
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