If it isn't working, find something that does:
- Screw your hands into the floor (visual and verbal cues work here).
- Pull your shoulders back and down (visual and verbal cues seem to work here).
- Pinch your shoulder blades (tactile cues work here).
- Break the bar on the way down and pull the bar apart on the way up (as it relates to pressing especially on the bench).
- Elbow pits forward (visual and verbal cues here).
- Elbows in or back (visual and verbal).
Remember that you are saving these young people pain in the future and teaching them to actively engage the proper musculature while preserving the soft tissues in the long run. Also, if results are motivating, that which is mechanically sound will ultimately lead to greater performance in the long run.