asked about schools in the UK as I remember watching a Jamie Oliver show
To be honest I don't personally feel that much has changed. In 2015 new school food standards were introduced. For example, one or more portions of veg or salad must be offered as an accompaniment each day, more emphasis on making water the drink of choice. etc. Here are some links:
In my opinion UK meals are still too high in fat and sugar, although there is far less salt than in the Polish school meal menu, and more vegetables on offer. The problem here is that although healthier options are available,many students will still go for the less healthy choices, battered fish and chips, sausage and chips etc. At break times it's easy for students to grab pizza slices or sausage rolls too.
Not all schools have meals supplied by catering companies either, many schools cook the meals on the premises. To be honest, I don't much like the idea of pre-prepared food which has been hanging about.
The problem in the UK is that it has now become cheaper to eat crap food, burgers, chicken nuggets etc than it is to eat healthily. Plus eating out at fast food places is the norm rather than a an occasional treat.I think this is reflected in what students decide to eat at lunch times.
I'm not sure why we're educating kids to expect dessert after meal
It was no different when I was at school, but I agree with what you say. When I was growing up we had dessert on a Sunday and that was it. However, for some students that school dinner might be the only decent meal they get. More than likely that thought was in mind when school dinners were first introduced.