As emergency extra bits in DD's schoolbag (no nuts policy in school - there are a number of DCs with allergies), she keeps:
A bag of popcorn (I get the 15g bags)
A bag of mini breadsticks (Tesco do a multipack of snack sized ones)
A box of raisins (matchbox sized)
A Nature Valley bar (currently a choc chip one, we also do oats and honey, or maple syrup versions)
The way I look at it is that she will only accept her hot food flask for lunch food. She does eat fruit but not at school. She does eat breakfast, but sometimes not enough or actually gets quite hungry at school or afterschool, and while afterschool give a snack, it is not usually substantial.
So she keeps a few bits that won't go off in her bag, and we check that every few days and restock as needed. Since going back, she's actually been eating quite a lot, but last term, she'd maybe eat the popcorn once a week or the cereal bar once a fortnight.
And the days that she hasn't eaten enough, she comes out in foul form and growls at everything. So I also keep single finger twixes in the car and some plain chocolate to give her a finger or a square to get her bloodsugar back up.