Hi
I have to say that the cast to HS in straight body is annoying to teach, because it does initially require a great deal of strength, timing and coordination. When you have all that down is becomes actually more about consistency through practice.
Personally if you are able to, teach the straddle cast to HS first, get the kids to be confident to hit HS, as well as learning not to push back immediately. Also teach them how to fall out of HS safely.
Along with that spend time working on catapults over the bar...
Put a soft crash mat on one side of the bar, have the kids cast but without trying to go to HS, instead teach them to cast over the bar and fall flat back on the mat.
(this is an example of the drill show on Tumble track website Link Removed ). I would recommend that you require and demand MUCH higher standard then that shown, in regard to form, shape, tension etc.. The arms bending should only be acceptable for the first couple of try's until they get the hang of it. After that BIG NO NO, learning bent arm cast to HS will only teach you a BENT arm cast to HS..fixing that after is worse then re-learning the skill.
I have also seen coaches be very successful with jump from spotting block cast and push back to land on block..With the no spotting approach. However i think there has to be a middle ground at best. Shape the HS always i think.
Things to learn and remember when casting to HS, is to push the hips into the bar (in order to have the bar spring you up), press the shoulders down (and keep them down till the legs start raising above shoulders), and then forcefully open the shoulders..this is where the strength component is needed and it helps to have learns straddle HS as this is a strength progression in a way, doing lots of planche lever-ups , all that sort of stuff.
I would definitely apply Dunno's progressively overloading methodology for training the cast, how you apply this is up to you...spotted, no spotted, you name it, but overloading is important...i think that is a big reason why it takes long to learn for most. Emphasise the tight arch shape in the cast at first.. once they can hit, and they get stronger this tight arch will disappear with a bit of work, but otherway around isn't so easy to do or learn.