DD is fussy but we have a healthy eating policy in school. She HATES sandwiches.
She has either chicken goujons, pasta with grated cheese or something similar in her hot food flask.
She has "emergency" extra food in her bag too - which she was almost never eating but is on a growth spurt at the moment so eating loads again:
15g bag of popcorn
Nature valley oat bar (can't have most types as there is a nut ban in school for allergies)
Matchbox size box of raisins
At the moment, because she is so hungry (and in Y5 equivalent, where they have a more relaxed attitude), she sometimes gets a fun size bar of chocolate or pack of crisps, especially on longer days (afterschool activities).
She will eat an apple walking to school in the morning but not in school itself. We've gone through many phases of eating dried fruit over the years - mango was the biggest hit for longest, but also apple rings, figs, and Lidl's dried fruit mix (had pears, apples, raisins, mango and other bits in it). We've also had periods where crackers and cheese were acceptable, so snack packs of cream crackers, Italian crackers and tucs have all been regular items on our shopping list.
She was never a fan of yoghurt. We had a period of lunchables (only a couple of weeks though).
Her water bottle has always been either plain water or fruit squash, and she has had periods of liking innocent smoothies.
I work on the basis of making sure there is solid food there that she will eat, other food in her bag that she will eat if she's extra hungry but won't go off if it sits there for weeks, and having the odd treat (pretty much the same as at home).