How are you spotting this skill?
I like to put one hand on their upper arm/shoulder to keep that over the bar, then I use the other hand to lift the back leg of the shoot through up (not back, and they must keep this leg straight or it won't work) as they press down on the bar and lift their front leg up and through.
I will also spot this skill from the front, holding on to both of the gymnast's shoulders/upper arms (on a short or extremely well matted bar, i.e. take height out of the picture). No matter what, I would hold her shoulders in place not let my gymnast drop her shoulders behind the bar. She would even balance there for a bit before putting her leg through. But eventually she got that her shoulders had to stay over/in front of the bar.
Also, you can do squat ons, as this too demands that the shoulders remain in front of the bar with the lifting motion of the hips/chest and pushing down on the bar. We often start them by putting a mini tramp in front of a preschool bar, jump on mini tramp a few times, squat on, and jump off onto a block about the same height of the bar. Make sure the block is big enough that they don't go flying forward and fall off if they goof up.
