Question

why do you think this happen if when throw food in our class dustbins

Answer 1

your classroom will have a bad smell + full of housefly near your dustbin

Answer 2

first of all, none should throw food anywhere as it is insult to the food. we are wasting food and many are there in the world who don't get food. so if anyone throws it, particularly in school dustbin, then that will lead to foul smells, bad sight in the class as no one will be willing to go near it and it can lead to attraction centres of flies.