There is an unwritten, but vastly vocalised rule in medical school. It goes something like this....
DO NOT WEAR YOUR STETHOSCOPE AROUND YOUR NECK WHEN YOU DO NOT KNOW HOW TO USE IT!
...and by this, I mean unless you can do more than listen to a heartbeat, you shouldn't be proudly displaying it like a badge, or oversized piece of jewellery. Because, let's face it, that's what it is.
Medical students in the lower years often have a day or two at the hospital every year, and every year you see them waltzing around the canteen with stethoscopes around their necks. Even the fifth years don't do this. This is why God invented pockets or small handbags.
Wearing it on a ward round, when walking around the hospital generally is fine, when you know how to use it, but the canteen is just wrong. It would be like wearing it while on public transport. It's not big, it's not cool and it's not clever.