Unfortunately there are a lot of parents out there that no matter what preventions the school puts into place will still be feeling them shit when they get home.
In some ways it's good that those children maybe getting their only decent meal at school - in other ways it's a shame they have to make the whole packed lunch thing so strict at some schools. I think it would be much better to have a word with parents of those whose children were eating Haribo for lunch 
I know my DS has a good balance. We eat lots of fresh fruit, vegetables, quorn, hummus, everything we eat is wholemeal - pasta, bread & rice, good quality meat etc.
However we also had a pizza takeaway last week with Hagen Dazz ice cream for dessert 
DS has treats sometimes but it's all about moderation. If he does ask for sweets he only gets a few or a small amount - so he wouldn't get a whole tube of smarties or a bag of chocolate buttons - he would get 1/3. Luckily he loves yoghurt covered raisins and considers these to be better than sweets.
We have a friend of the family who feeds her children utter crap - always has since they were toddlers - SAHM with plenty of disposable income who feeds them frozen pizza, sloppy Joes, beef pancakes, turkey dinosaurs with chips etc EVERY evening meal. The children are exceptionally fussy even now at aged 6 and 8, only eat junk food - are constantly ill and always have a 'greyish' complexion.
It's such a shame as with a lot of children these eating habits will continue into their adult life as they know nothing else.