Working on good form and not drifting when jumping. Penetrating the net with their hands and blocking the ball towards 6 on the opponent's side.
The feeders should feed at the blocker's pace when starting and increase pressure as blockers become more competent.
With older players you can add an exercise for players to complete after every two blocks (ie a press up). You can also increase the number of blocks each player must complete before players swap roles.
New hitters rarely miss because of their arm. They miss because of their feet. Build a reliable approach, then time it with your setter.
When your hitter gets stuffed, five teammates standing and watching is how you lose the point. Good attack coverage turns a blocked ball into a second swing.
Coaches spend hours correcting arms in serve receive when the real fault is almost always the feet. Fix the movement and the platform fixes itself.