Ahhaa!
I knew it! (no hindsight bias here, as I will explain)
Well, you see, I pretty much avoid sweets of any kind (unless it's chocolate or coffee-flavored..but that's about it). They enacted the same restriction/law in my school, too.
When I stopped eating sweets (@age 7 or so), I pondered...what exactly is bad about high sugar content in foods? Is it better or worse than high fat content?
Without sweets, I began to eat much more. Much more. My appetite expanded CONSIDERABLY--that is, for filling foods: plenty of burgers, bowls of fried rice and chow mein, burritos...etc (cleaning out buffets!). But I stayed a lean ectomorph under 140 pounds (somehow). Generally, I took six to seven mile walks around the city (when bored) and seemed to have limitless energy--until high school's homework kicked in (essentially pointless--I get A's on tests and am the most academically involved student in class (not @home)--why the required+tedious homework?) and I got about four to five hours of sleep.
Then my school enacted the law/restriction on "junk food." Until then, my lunch meant Doritos (registrated TM) and a bottle of Coke (registrated TM).
My diet was fundamentally three rules:
1) No milkfat!--unless it was milk (which I drink quite a lot of) (or some coffee-pie/etc)
2) No sweets!--unless it was coffee-pie or something (not that it exists, tho)
3) Rule of thumb: no large amount of fat unless it came from something strictly filling (like a burger, burrito, nice fish, giant red steak...etc) or a healthy source (olive oil) or it was required for the meal (like sour cream for beef stroganoff)
And I stuck with this diet (no butter in oatmeal breakfast, no butter in instant rice/potatoes, no mayonnaise in anything...etc)
Basically, I ate...and ate quite a lot (large bowls of chow mein, several burritos, many burgers, other meals fit for three (sorry, no objective standard established behind this statement) but stayed slim (somehow!))
Next, (and randomly throughout school) came texts on what not to eat...etc. This meant no burgers/fries/sodas...etc. I don't really drink soda or eat fries any more (unless I happen to go to a burger place), but the criticism of burgers seemed somewhat peculiar. Suppose there was NO mayonnaise (outlawed by my rules) the patty was well-done in a cooker that eliminated much of the fat. In fact, suppose there was no patty--just some lean bologna (now a sandwich)--that would be healthy. I pretty much avoid sweets altogether, but stick to my rules 93% of the time.
But this is where my sugar question comes in. Now there are healthy foods even with high fat content (I think...but mostly low fat content), but none (almost none?) with high sugar content. Hmm--is high sugar content somehow worse than high fat content? What are the risks to high sugar content?
Thus, I decided that "junk food" must then be completely unhealthy (i.e., no significant health benefits whatsover). And I then thought high sugar content must really be the condition! Furthermore, my long story in this post is not required!--but I wrote this as a minor nutritional biography. My motives originally were to objectify the standards by which a food is deemed "junk," and I elicited a response of high sugar content--pretty much what I expected, or hoped to expect!
*Nevertheless, however, perhaps high sugar content is an oversimplification---(or maybe not..). Are there any other criteria, exactly (perhaps), that characterizes "junk food"? Are there any other conditions that must be met to deem a food as "junk" ?
we get to squirt our own katsup and mustard on our food! 
And we can go wherever we want (within the school) until next period.
Next we plan to raise the driving age... 
What are all your parents doing? Why do they let them bring a bunch of junk to school instead of a healthy lunch? How can kids pay attention and learn if they're pumped full of sugar and caffeine rather than nutritious food?
