As a teacher who's done her fair share of eating duty with the preppies, my suggestion is to make their food count. As a PP suggested, they're often too busy talking to eat, so what little they do get, make it 'good'. Make it 'finger food' that's easily opened and eaten by themselves (lining up to have your muesli bar unwrapped with 15 other kids means a long wait and a frustrated teacher, lol! Or, the yoghurt packet that squirts everywhere or rips apart etc) or quickly opened by others (e.g. one container with 3 different sections as opposed to three separate containers). Pack whatever normally appeals to them at home, about the same amount plus a little extra for 'luck' (or spills or a ravenous appetite or 'shared' with their new besty etc).

And have something on hand for when you pick them up - as a pp said, they could be ravenous!

Make sure they know they don't have to eat it all, and suggest they bring home what they don't eat, at least at first, so you can see how much gets eaten and pack accordingly.

Is there a fridge to put lunch in? If not, even with an ice brick Krysalyss, stuff gets hot and festy really quickly around here, so I wouldn't be packing stuff that could give food poisoning...