Some guidelines for Halloween costumes.
Blackface costumes (including being “thug” “ghetto” etc.) are never okay (http://en.wikipedia.org/wiki/Blackface) no matter how many “friends” you have who are Black, “from Nigeria” etc.
Similarly, dressing in other ethnic costumes such as being Mexican, Chinese, etc. is not okay.
Even if you are from a particular background, this doesn’t excuse you from being racist. For example, a Muslim student wearing a turban, a beard and imitating a “suicide bomber” is as uncool as non-Muslim student doing the same. Possibly uncooler, actually.