don't worry about your looks
Even the most beautiful people grow all wrinkled and shrunken after a few decades. What really counts is the character because that doesn't ever wrinkle. I'd much rather spend my life with an ugly but kind and gentle man than with a handsome egomaniac.
I'd still try that smiling bit, though
Most people look a good deal better when they smile.
How about telling your pupils why you don't smile much and how having been bullied does still affect your life? They would begin to understand you and that would make them stop being afraid and start liking you. Plus they'd begin to think about what bullying can do to people and how it can be prevented.
You could use that anti-bullying video by breaking27 as a starting point to introduce the topic and then explain to them your personal experience. This would make them less inhibited and they'd perhaps start to talk about their own experiences. This way you could protect your pupils from having to make similar experiences as you had.