Quick Tip: How to Use, Clean, & Store Kneaded Erasers
Kneaded erasers are easily the most versatile of all erasers used in drawing. While many erasers are only really able to be used with one format (ie: white plastic erasers should only really be used with graphite), the kneaded eraser can be used on many different 2-D mediums, including graphite, charcoal, pastel, and colored pencils. (Note: Any eraser that leaves tiny 'shavings' on the paper should not be used in a charcoal composition. It might cause you to smear your work!)
Kneaded erasers are square when you first purchase them, but do not maintain that shape for very long. The appeal of kneaded erasers is you can adjust just how much surface you want to erase by pinching and molding the kneaded eraser with your hands. (And since we're all creative types, you'll probably end up molding it into all sorts of mini sculptures during boring lectures.)
While other erasers tend to wear down and get smaller over time, kneaded erasers more or less remain the same size. However, the graphite/charcoal/pastels you are using will begin to collect in the eraser, which will require you to clean the eraser as needed.
You can clean your eraser by breaking it into halves, layering and molding them on top of each other, and then breaking them into half again. Continue doing this breaking/molding pattern until it appears clean again.
The one downside of kneaded erasers is that they tend to be highly sticky when they're in your art storage box. They can pick up pencil shavings, paper clips, and other small debris that could be floating around which can ultimately affect your eraser's shelf life.
What I suggest is finding two (clean!) plastic soda caps and sticking the kneaded eraser into one and 'closing' it in with the other. The kneaded eraser is sticky enough to keep the bottle caps in place until you need to take your eraser out and use it again. Now you can put it back into your art box and not be worried about it touching anything else!