Standing on the attack line in pairs, one player underhand throws the ball high, and the other swings their hand back and spikes the ball into the net.
Setter must throw the ball high enough so the spiker could prepare (backswing, rotate torso, step forward with hand-opposite leg) and hit the ball at the highest point over their forehead.
After a few attempts have players jump prior to attack.
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.