well, had a think and my views....

my children have/had school dinners at primary school. because one wouldnt eat a packed lunch at all, and i personally know the cook and dinner ladies so therefore there was no "bad dinner lady scenario". they tell parents if they have not eaten much on a regular basis and are not forced to finish. i.e, you get told if you are wasting your money.
mine then had a light snack when coming home and a cooked tea later on when dad gets home. this works well for our set up and i am not competing with the cheese stirng and lunchable brigade, because i wont by them!!. although saying that our school is now healthy and there isnt supposed to be any crisps, choccolate, lunchables etc etc.

my oldest now takes a lunch box to secondary school because there is much less control over what he can choose , i.e be could have burger and sweets everyday which is not good. so he packs a lunch. sandwich, fruit, crisps, rice cakes etc.etc

we have a snack time at preschool that is around the same time that the local school have morning break (which they now have to take fruit!) i think that this works as it does not interfere with lunch.
i dont think that 2 dinners a day makes chi;ldren overwieght but what the dinners are made up of.
many schools now are giong healthy eating so this will help, but it is the junk after school that does not help and the tv, playstation culture that we now live in. they need to paly more outside running around - like i am sure we used to

sorry if i have gone ona bit!!:)