1. Give that student a really good and powerful motivation. For example, don't ask him/her home task or just don't give a bad marks for a week.
2. Make that student to understand that in school it's not possible to be dressed like she or he dressed. Let that persone to realize that she/he can wear his/her unusual clothes somewhere else but not in school.
3. Encourage that student somehow. Give him/her a reasone to do all homework. For example, you can give him/her a longer break.
4. Talk to that bully, explain him/her that it's not the rightes way to say "Welcome to our class!"