“Hey, Nice Skin”

The Journal of Investigative Dermatology recently published that teenagers with severe acne were twice as likely to develop mental health issues as compared to the normal kids. Most commonly, depression. It’s a pretty strong statement about the importance we place on appearance during those formative years.
So, next time you see a teenager with severe acne, try and brighten their day.  Smile and say something kind.  "I like your personality," for example.

Also, as per this article from Health.com, it's best to not insinuate that their bad acne is an attempt to punish you. Even if you're pretty sure it is.