Question

Some students continue to litter the classroom during lunch break despite repeated warning by th class monitor

Answer 1

1.Let the class be dirty and let them experience sitting in a dirty class know it's I'll effects (learning the hard way)
2.Impose a fine of any sort not necessarily money but some other medium. (donate the articles gathered)
3.Arrange a litter cleaning time of about 5minutes during the last 5minutes of recess.
4.Try talking to them.
5.Beat them up.
6.Confiscate their lunch boxes!! XD
That's all I can think of. Hope it helped. Lol. :D