College Board has posted the official answer, and the short answer is yes, you can use I on your essay. To paraphrase what College Board says (you can read the page noted above to confirm, if you like): since the essays ask you for your opinion, you are permitted to use the first person on your SAT essays. Many SAT test takers are afraid to use the first person on their essays because they've had it drilled into them since they first learned writing that in formal essays the writer should not use first person. This may be true for the most formal of writing, such as that you would find in scientific journals, papers presented at conferences, or in legal documents, but the truth is that in modern American writing, hardly anybody uses formal English in their writing. Take a look at even the most highly-regarded newspapers and magazines, and you will see that most writers today use the first person (I or we) or second person (you) quite liberally.

So, don't get bogged down on this point; focus your energy on more important things, such as developing a great story.