I also have a DD who is 2nd centile for weight. She has always been slight and has to take medication which has an appetite suppressant effect. There is a healthy eating policy in school.
BUT IT IS SENSIBLE!!!!
Nothing is ever confiscated.
The school raise it with parents frequently about having healthy options. I wanted to go to the talk tonight about healthy lunches with a dietician - but DH has to go to the talk at her sportsclub which is mandatory. (I might just bring DD to school and go anyway). They have done the "Food Dudes" programme in Y1 equlivalent, to introduce them to various fruits and veggies.
The idea is to have some fruit or veg, and enough decent food to keep them from being hungry daily. They are allowed have a small treat on Fridays. HM cake is allowed (I get the impression it should be plain, not iced). The afterschool club has hot soup, toast or toasted sandwiches most days when they get in (in school), there is fruit available, and they get a slice of pizza each on Fridays.
DD hates sandwiches that are not "fresh" (made there just before you eat them) - so refuses to eat sandwiches for school lunches. We have tried sooooo many things to get her to eat. Currently, she alternates between a flask of tomato soup (she may drink a cup) or her food flask with some hot chicken, hot pasta, hot potato wedges, or hot leftovers from previous night's dinner. I always have a second tub in her bag with a small tub of raisings and a chocolate covered rice cake - in case she actually feels hungry. I often send in things like tomatoes, carrot sticks, pepper sticks, cheese, pieces of cold meat, snack packs of crackers, small tub of tuna mix....sometimes they're eaten, others ignored. With fruit, its the same - she might eat grapes or a chopped apple, or green banana - or ignore them.
She will eat more at home - mandarins, lots of raw veggies (tomatoes, carrots, peppers, corn etc), a fair amount of cooked veggies (not everything, but mushrooms, peas, carrots, cauliflower in cheese sauce, broad beans etc), and a reasonable amount of dinner usually. Somedays, she will have a couple of spoons of cereal for breakfast - others it will be a full bowl and a muesli bar and fruit; or scrambled eggs with ham, cheese and tomato; or a couple of sausages; or a tuna melt; or have a chunk of cheese to nibble while going to school.
There are some girls in the class (Y3) already talking about being fat and watching what they eat. DD doesn't talk about that at all, and will say she doesn't want meds at weekends "because she is starving and needs to eat loads". Recently I have started giving her 300ml of hot milk going to bed (we have smoothie bottles which are handy for non-spill in bedroom) and that is helping both getting to sleep and with increasing calories.
Actually, the more I think about it, the more I NEED to go to the talk tonight.