Why do guys looks at my chest?

They're in eighth grade, which means they're just starting to notice girls, probably, and most girls are just starting to grow. They're bound to be curious about this new addition to your figure and they might not really notice they're doing it. As long as they don't touch, I wouldn't worry about it. If it really makes you uncomfortable, when you notice them looking, just cross your arms and they should get the message and look back up.