I am not a student, but have only been out of RN school 4 years. Hope you don'tmind my answering.
I teach NA classes, CPR classes and Teen Volunteer classes at my hospital. Usually what they want to hear, if you are free to choose any subject at all, is "What is it REALLY like to be a nurse? What is a typical day like?"
Wow...you can talk all day about this and it lends itself well to a novice speaker. You always want to talk about something you really know.
Has the instructor at the class given you any guidance? You may want to ask what they want/need out of you, time constraints, etc. What kind of experience have you had in giving talks/speeches? You must have impressed this instructor to have had him/her ask you to speak!
If you need any hints, I'd be glad to help! PM if ya' want.