Ideally all arrows will be longer than the class requires. Short arrows can be used by the wrong child. Teach them to rotate their bow elbow to the outside. It prevents bruising. Keep extra long armguards for some.
Some arms overbend naturally to the inside.
Watch them as they shoot. Try to coach each one as you can. Some will need you to check for dominant eyes. Have them hold both hands out in front of them and make a small window, then bring their hands back to their face while they are looking at something. That will tell you which is their dominant eye.