I wonder about that, too, perplexed. I have 2 boys with ASD and some severe food aversions tied in with that (including gravy, boiled rice, boiled, baked or roast potatoes, savoury pastry and custard, for DS1 - I can't even pay him to eat those things - and almost all vegetables and raw fruit for DS2). Added into the mix, we've got DS1 on an exclusion diet for probably abdominal migraines, which includes chocolate, cheese and citrus - that means he couldn't even have the veggie option, most days.
As for the social side, DS2 sits with a bunch of friends, who help him open things and chat with him (he's non-verbal) and they all eat their packed lunches together. DS1 can't tolerate the noise of the dining room, so eats his packed lunch in a quiet room with some staff and sometimes a friend.
They're in juniors, anyhow, but compulsory school dinners would be discriminatory towards them both and would mean that their diet was worse. DS1 would probably not eat at all, if he wasn't allowed to take a packed lunch in. DS2 would eat very little, as the portions are tiny, even without the high proportion of the meal that wouldn't be eaten.