It's actually quite surprising just how fine the line can be between what is considered a joke and an insult, even when the perpetrator is a so called adult.

What may seem a jolly little jape to one person can be surprisingly hurtful to another if it hits a sensitivity. Unfortunately teenagers have not, in general, got to the point where they can think things through to the ultimate conclusion, nor are they really aware of just how much their fun can impinge on others. Mostly, IMHO these kids are not being racist, sexist or any other type of ist - they are generally just performing for an audience of their peers and trying to make themselves look clever.

The shame of it is that some people just never grow out of that habit and, even in their so called mature years, don't get that line, or don't consider that it should apply to them.