Is calling someone really skinny a compliment or an insult ?

It was a compliment, I'm sure. Some people say it, not thinking someone could be offended. Being skinny is something most people desire.

It depends on you. I'm sure your teacher was just stating the fact that you're skinny and whether you consider it a compliment or an insult is up to you. Unless your teacher said it in a mean tone or during an argument, your teacher was just stating a fact!

Depends how you take it I hated it becuase I would go to the gym and didn't want to be called skinny but now I'm toned so it depends who you are

I think your teacher is concerned about you. Teachers don't usually compliment their students about their body.

I would say it was most likely a compliment.

an insult hunny. At that weight, she's probably concerned your parents ain't feeding you right

It's damning with faint praise