The trash being left out is caused by kids who are now lazy and expect people to clean up after them like their parents do at home. they expect all their things they need to do to be done by other people. kids can no longer go the extra few steps to throw away a rapper, instead they figure it will be fine to leave ONE rapper on the ground. but that one rapper influences other kids to do it so then after one kid does it, then another, and another, and another kid does it it eventually builds up to A LOT of trash on school property.
hope this helps!:) (just examples of what you can say)