I think it's a question of degree. Teasing is meant to either annoy or playfully mock the recipient. It can be either playful or mean, and sometimes gets taken to extremes, at which point it's no longer teasing, but abuse. Bashing, however, is aggressive and threatening, the verbal equivalent of getting severely beat up. In the middle is mocking, which, to me, is like heavy-handed teasing with malicious intent. Bullying is a means of intimidation and usually carries a threat or undercurrent of violence.
Wikipedia offers an in-depth look into bashing and its implications.
While writing this damn long comment, I consulted a dictionary, and this is what it told me:
Tease: "1. To annoy or pester; vex. 2. To make fun of; mock playfully."
Bash: "To engage in harsh, accusatory, threatening criticism." Also "to beat or assault severely."
Mock: "To treat with ridicule or contempt; deride."
Bully: "To treat in an overbearing or intimidating manner."
So yes, I think it's a matter of degree and intent. The difference between teasing and bashing is much like the difference between laughing at someone for the way she's dressed and punching her in the mouth because of it.
It seems that the question becomes, when a person or group is engaged in an activity that mocks or saterises another person or group-what is their intent and motivation, and will it be hurtful and truly harmful for the people being mocked, were they to experience it?
I think that the lions share of responsibility for determining if something is harmful is first, the recipient or subject of the mockery, followed by society as a whole. First for the subject, to stand up and say what's being done is hurtful, and should not be accepted, then for society to agree, and take steps to eliminate the mocking.
For example, take blackfaced performance. Up until the mid 20th century, blackface was considered and acceptable way to portray African-Americans. No one though twice about if it was offensive or harmful. Even some African-American performers wore blackface when portraying black characters. It took years before that form of mocking the apperance and stereyotyped personality of 'coons' was deemed unacceptable.
Obviously, offending someone with mockery or sature is unavoidable, because of the nature of satire, but when does mockery cross the line, and how do we determine that? Just saying "X deserves to be mocked" is unacceptable, because it runs perilously close to telling someone to just get a sense of humor, or you're 'just joking'.
Tell that to the Jews trying to eliminate the mockery of the 'moneylender', or the Pagans and the warty witch at Halloween, or the Christians trying to get out from under the shadow of Phelps and Robertson, and so on and so forth...