Longer tubing into the binaurals to cut down on having to bend very far. A good, tunable diaphragm on the chest piece. And sturdy construction. Brand names most often recommended are Littman and Welch Allyn.
The most popular brand of stethoscopes for students in the medical field is the Littman brand. They market to all different countries and are popular for use in college as well. You should buy a stethoscope that the student is already familiar with. If the student hasn't used any brand before, you should ask the school where the student attends and see what brands they use. Choosing a stethoscope that is the same brand and model as the one he/she uses in school will make it easy to familiarize with for real work use.
A good stethoscope for a vet student will differ from one for a doctor. You should look for one that has extra long tubing. This will make it easier to use on larger animals. Getting one with a bell would also be good. This will insure that you can hear lower frequency sounds which can be diagnostically important. It's a good thing to try before you buy just to be sure you get a proper fit and it has everything you need.
You want to be sure that the stethoscope fits correctly. If you can't get sound past the ear tip to your tympanic membrane, all other features are unimportant. The more custom the fit, the easier the sounds travel to your ear. You should also make sure that it fits comfortably in your ear as well. You also want to make sure that the one you get is rugged and durable as you will probably be using it for quite a long time. You should also check the clarity of the sound, if it does not have clear sound then you do not want to get it.
You should match the stethoscope to the ones used in the classrooms. This will ensure that the student you're buying for has practice and experience with it. It'll also be easier to learn to use in the real world job. You should check out all the brands and compare prices before choosing a stethoscope when shopping on your own. Do some research and see what vets are currently using in the industry.
The stethoscope that you should get should be high quality and not cheap. You should train with quality materials because it puts you at an advantage. Training with bad materials would no give you the best education that you can get.