Instead of submitting a copy of your teaching certificate with your application, you may submit verification that you are matriculated in a certification program (e.g., letter of acceptance, letter from advisor).
Expository writing is writing that seeks to explain, illuminate or 'expose' (which is where the word 'expository' comes from). This type of writing can include essays, newspaper and magazine articles, instruction manuals, textbooks, encyclopedia articles and other forms of writing, so long as they seek to explain.
The purpose of an informative essay, sometimes called an expository essay, is to educate on a certain topic. It is not for giving an opinion or convincing someone to do something or change his beliefs.