What should be in the school tiffin so it actually comes back empty?

Practical constraints matter more than nutritional ideals: it has to survive four hours without refrigeration, be eaten with limited time and often without cutlery, and not embarrass the child. Things that generally work: parathas rolled with a filling, idlis, besan chila, upma, cut fruit that does not brown quickly, and roasted chana or nuts where the school allows them. Involving the child in choosing within a set of options substantially improves the return rate. Avoid anything that leaks, needs assembly, or is a new food being introduced for the first time at school.