HS Principal to Students: No Leggings unless you're a size 0
On a side note: The entire sexy at any size body positivity movement really isn't healthy. It is not helpful to tell people they are still sexy when morbidly obese.
Sexiness and healthiness are two different and separate things. Health can be measured through the collection of impartial data of one's body. Weight is a factor in that measurement and usually a predictive factor, but not the only factor.
Sexiness, on the other hand, is entirely dependent on the individual person and their experience, culture, frame of reference and attractions as to what they consider sexy. While you may not find that morbidly obese person attractive, I guarantee you that a not small percentage of people do. I think it's typical for people who meet or believe in the popular standard of sexiness (which also correlates with healthiness here in the US) to believe that they are both the same thing.
In that sense, I think it's perfectly fine for a fat or morbidly obese person to feel good and positive about their body and feel sexy in it. And I think it's fine for people that are obese or morbidly obese to wear whatever makes them feel sexy, they don't need anybody else's approval. That however doesn't and shouldn't take away from having honest, frank and important conversations about health and the effects of added weight on a person's health, especially with health care providers. We just don't need to tie it to sexiness and attractiveness.
My general advice to everybody is to dress appropriately for whatever situation you're in. If you're going to a club, wear your sexy arse clothes, whatever that is to you and don't give a flying fork what anybody else thinks. If you're going to school, wear something more modest. Be your own clothing editor so others don't have to be.
I happen to think body positivity is a good thing because it allows people to take stock of their own health and work honestly through that to hopefully better outcomes without being encumbered by the judgement and negativity of the external world around them.