Answer:
Dear students,
I urge you to take care of our school; pick up food packaging, dispose of trash in bins, and tidy up after yourselves! Littering seems to takes place during lunchtimes and breaks, so activities such as eating should be done with care; dispose of your crisp packets or bar wrappers in bins. Littering has a negative impact on the literal school environment as well as the atmosphere - no one wants to come to a dirty school!
This is a (rushed) 80 worded letter, but you can edit it.