Your first reaction to such situations should be a quick one but with proper planning. Make those students understand the fact that throwing food is not at all a good thing. If then also they don't understand then share this problem with your teachers and then ask them to conduct some plays in the assembly for spreading awareness regarding the situation mentioned above. Also tell them the consequences of this like health hazards etc.. You can interact with your partner by taking the help of the info. I added in the answer itself. Conclude the problem with a proper solution in your dialogues.