Bad posture is not just a matter of slumping. It is not entirely caused by laziness or weakness of back and shoulder and Abdominal muscle groups. Bad posture is a self defense mechanism.
There are people who draw attention to themselves, stand out in a crowd. They walk with a Purposeful Stride and people move out of their way. They walk with their shoulders back, drawn to their full height, their head up so that they can look around as they walk. They will look you in the eye if you give them a chance, so you look away before they do. Before they can catch a glimpse of what is beneath the surface of your skin.
You don't want to be noticed. And something has to be done about that unasked for height, the strange broad area of your chest and stomach. So you slouch. You bring your shoulders down and forward to reduce your frontal area. Create a cup of your chest, shoulders reaching toward each other trying to meet. You cross your arms. There is less to see and so less looking is done.
So don't just walk, you sidle, glide. Trying to create as little disturbance as possible in the air around you. Maybe you won't be seen at all.
You slip into rooms and sit at the back, on the outskirts. Your pelvis turns up, rotating in on it's self. Your back goes Convex, swiveling to protect the front. Your shoulders scrunch towards your ears. Your chin goes out, head down, stretching your neck towards the ground. You can't actually lie down on the ground, because this would bring too much attention, this would be causing a scene. But being so close to the earth, feeling the dirt under your fingers, singing up through the floorboards. You could almost disappear into it. Instead you spend your time engrossed in it, dreading the possible encounter.
Teachers and mothers used to tell you to sit up straight, not to cross your arms - you'll look surly, and you looked at them with a pained expression. They want you to give up your only line of defense. You'd rather take the ruler slap in the hand, the poke in the ribs.
All you can think is "Don't make me be seen. Don't make me be seen."