What you'll get with a lot of kids is, if you give them a tee drill that asks them to hit the ball deep in the zone, they'll cheat the drill by moving their feet back to accommodate their bad habit, essentially striking the ball at the same point in their swing as before.
To combat this, I use a double tee drill. Set up balls on two tees at the same height but 4-8" apart. Then set the rear ball deep and the front ball where they want to hit it already. Then make them swing through both balls.
Another thing you can do is set something behind the rear foot to prevent them from scooting back on you, e.g., a cinderblock.