if it's only regarding the invention of a specific dental equipment, it'll be to inform. the student is preparing to share their knowledge on the dental drill.
to persuade is usually the purpose when there's a question or an argument being raised, and in this case, it doesn't sound like anyone would be against dental drills. dental drills is also a fairly dry topic, so it's doubtful that the student would be aiming to entertain. it isn't a demonstration because the student likely isn't giving a step by step how to in regards to using a dental drill.
