Pre-IB Essay. Help, Help, Help!?!?

Oki. Well, I am in grade 8 and I am applying to be an Pre IB student, thus, I need to write an essay, saying why I want to join IB. This essay should be 400-500 words, However, I do not know wether this essay should be in MLA format, or in a simple persuasive format. Hence, my question is, What type of format do I use for this essay. PLZ HELPP!?! =)