I do a mix of things in a Primary school. Recently I've been supervising the lunch hall.
The food meets the standards required eg puddings are sweetened with fruit, fried food once a week etc.
For KS1 (universal fsm) I estimate that I throw about 75% of the veg/salad in the bin and a varying amount of the protein/carbs - I'd say on average 20%, mostly the known choosy eaters (I'm not privy to all the children's info so don't presume that all choosy eaters are NT) and a small number of very slow eaters. We do our best to encourage the children to eat but we have a strict half an hour for them to eat and leave so we can clean up and get ready for KS2.
Children crying over a plate of food they don't like and won't touch happens (don't talk to me about the day they put stuffing in the roast chicken roll) with the child usually telling us that they told their parents/carers they don't like it (take that with whatever size pinch of salt you choose). We are not going to try and force a child to eat and the school doesn't ask us to but we do praise trying new things and having a go.
Since the portions are calculated to feed the average needs of a child anywhere from 4 to 11 - the kitchen staff do skew this a bit - plus choosiness (whatever it's cause), some of the children are undoubtably hungry by the time they go home and some when they enter the classroom for afternoon lessons.
We throw much less food away for KS2 as most of them only have it because they like it. I used to be in favour of FSM for all but now I think, on balance, I am not in favour of universal FSM for even KS1. I do belive the threshold for FSM should be greatly lowered so more can get it or perhaps a voucher scheme that can be used for school meals or food shopping.
Whatever we do, with all the good intentions in the world, schools trying to feed 100s of children may not be best placed to get individual children to eat at all, never mind healthily. I say this a someone who got FSM as a teen and was profoundly greatful for them as they were almost my only source of vegetables.