Congratulations on being in nursing school. It is very difficult as you know to get in, but enjoy it! As for the stethoscope, a few things to try before rushing out to go buy another one. Make sure the ear pieces are sitting properly in your ears. May be difficult to understand, but pull out the manual that came with it to get a better understanding of this. When you put the earpieces in, they should be angled towards the direction you are looking. On the Littmann, they are already pre-angled, do not try to adjust them. Take the head, (diaphragm and bell) or the piece you hear with, in your hand. With the earpieces in, the proper way, lightly stroke or tap the diaphragm (plastic piece with the "L" on it), and you should be able to hear sounds. If not, rotate the head until you feel a "click." You will not break the head by rotating it, it is supposed to do this. The bell (curved shape) works on hearing low frequency sounds, and the diaphragm works on hearing high frequency sounds. Finally the diaphragm on all Littmann's is pressure sensitive, meaning the more pressure, or less pressure, you will hear different sounds.
This may in fact be the issue, but perhaps not. Learning how to check a BP is difficult at first, but after many, many times, it does get easier. I, for one, do not care for the Littmann Classic II because everything I heard sounded muffled and diminished. I do however use a more expensive Littmann in my daily practice, but it is not necessary. Only reason I use something different is for the mere fact I received it as a gift. Many stethoscopes out there and many will say, it is not about what you put in your ears, simply what is between them.