Are they able to hold hollow on different types of surfaces? At our gym we hold pushup position (hands on a floor bar) with feet on a small barrel or athletic ball. Then we rock forward and backward, emphasizing flat hips. Students are challenged to lean as far over the bar as they can when they rock forward to get used to the feeling of having their shoulders over the bar.
Here are some other "fancier" setups that can drill casting shapes (and can be easily modified to the student's current ability):
Two other things to consider:
1. Are your students scared? Sometime bad casting comes from a fear of landing hard on the hips or a fear of falling forward. Achieving correct form can actually be quite scary for beginners. Have your students practice on a bar that's close to the ground and add some padding on the bar. That might help them work up some bravery.
2. Is shoulder strength the problem? It's great to "work core religiously," but for casts, you also need shoulder strength to maintain tension on the bar. Perhaps your students need more upper body conditioning? I recommend shoulder shrugs: on a single rail, between parallel bars, in a pushup position...anything to get them thinking about shape change with respect to the shoulder blades—elevated vs. depressed, pinched vs. rounded.