Sally - Anne, I don't understand why you throw your waste papers on the ground?Anne -  Sally, you know that the bin is so far away from our desk. Who would suffer walking from here to there to throw a piece of paper?
Sally - I would.
Anne - What do you mean?
Sally - Anne, we don't want youngsters to learn from our clumsy behaviour, right?Anne - Yeah, so?
Sally - So when you're littering the place, young ones are learning from you. And then even the two or three people who try to keep their surroundings clean, will also fail in their decision.
Anne - But, what does that matter to us?
Sally - Their surroundings are also your surroundings. Do you want to live in a dirty, polluted and unclean country?
Anne - I understood your point, Sally. I'll never litter the area around me. Even if I have to walk to extra steps to reach the bin!

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First of all warn him/her from doing any further litter.
Then I would explain him/her the importance of cleanliness in class.
Still if he/she is doing I will have to complain to teacher(as in schools they are the head) and write a letter to the Principal on requesting to interfere into the matter.