First, you’ll want to attach the teeth to the mouth. One way to do this is to use the Face Landmark feature to pin the teeth exactly to the face. Alternatively, you can use a Head Binding to roughly attach the teeth to the mouth. 

Second, cover up the teeth with the face by using a Face Mask. Make sure the Face Mask object is rendered after the teeth image (e.g. the Face Mask comes after the teeth in the Objects panel) by placing it lower in the Objects Panel. Then, enable “Use Orig. Face” in the Face Mask option so that it renders the face rather than a texture. Next, set the Texture field on the Face Mask to use Device Camera Texture so that it displays the face before the teeth attachment.

Lastly, and optionally, you’ll want to disable the teeth when the mouth is closed in order to avoid them appearing when lips are closed like this:

You can use Behavior script to Set Enabled an object when the mouth open and closes, OR use the Face Landmarks to more trigger this when landmarks align.

Take a look at the Face Landmark template to see some helpful scripts that will allow you to attach objects to a landmark, as well as trigger effects based on the distance of the landmarks.