Select the head, switch to Edit mode, hide the head itself, select the teeth, go into the Object Data > Vertex Groups panel, remove the vertices from all groups:
Now in the list select the Head vertex group and click on Assign:
For the tail, it's not clear what you want to do, in any case you need to loopcut your tail otherwise it won't be able to bend:
You could use an IK constraint on the bone called Tail bone.026 and make the Tail Tip the controller, in that case unparent Tail Tip. Or activate the Auto IK option and move Tail Tip to move the tail:
But for a tail the best is probably to use the Spline IK constraint as explained here.