Think about this; if the teeth are poking through the skin of the face then the problem really isn't with the teeth, it's with the skin and probably the blend targets that were created. As the skin deforms the teeth poke through. This means that the face is deforming in an unrealistic way. Now, if your goal is to create cartoon-like exagerated expressions then you might want to use set driven keys to control the translation of the teeth but if you are trying for something more realistic then you need to take a look at remodeling your blend targets. Typically, the teeth (and gums) are either parented or constrained to the skeleton as explained by kbrown.

Try moving your blend targets into position over the teeth. If the teeth poke through the skin, use the soft modification tool to pull out the cheeks and lips a bit...