By doing the following things
making strict rules
keeping dustbins in every class
and spreading awareness to the students
swacch bharat campaigna and etc

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The best ways are=

Stop littering! this is the main cause of untidiness at a school.

in a classroom throw the wrappers, waste etc in the dustbin.

do not draw on the walls.

stop the banana peel issue

if any garbage found on the floor, throw it in the dustbin

These are my main points...........