I know exactly what you mean about the teenage girl comment. I'm dreading having to raise Brianna through her teenage years with society the way it is about plus size women. Being on the bigger side myself though I'm hoping it will help her realise that being a size 4 isn't everything and it certainly doesn't make you popular, happy or beautiful.
If the focus healthy eating and appropriate exercise in schooling continues as is already is in schools now, it will be much easier for kids to stay skinny.
For young people, most of which have a fairly decent metabolism, it shouldn't be too difficult to stay at a healthy weight. Some will be skinnier than others, sure, but if kids are armed with a good knowledge of nutrition then the battle to be 'thin' won't be a battle at all.
I wish I knew then the things I knew about food now. My parents always meant well and made sure I had plenty of food but it was largely the wrong sorts of food at the wrong time of day. Luckily, I was a pretty active sort of teenager, so I was relatively skinny but had I known a little more about how to eat, I'd have probably been a bit happier with what shape I was in.
Nutrition should be a mandatory subject in primary school... probably in high school too.
Before I start banging on again, I'll just say that being a healthy weight should be easier for the next generation of teens than the last few generations. We know much more about what we're eating and talk of nutrition is everywhere. I know that my parents didn't have much of an idea about it... it was always the 'growing boys' attitude to food and a mother's need to spoil. So long as the next round of parents know a little more about nutrition, the kids will be better off than ever before.