How to walk feminine at school?

Ok so my mom an sis said I walk hunchback which I don't, it's just a slight slight hunch and that's bcuz I think I'm standing straight buri don't realize it. And my mom is like u should walk like a model cuz ur skinny. But it's school. And isn't it a little exagerrated to walk like a model at school. How can I walk more feminine. Oh and I always want to sit up straight but I either get too tired or uncounsiously go bak to my original position, how can I sit up straight. This also happens while standing