No re-entry is the controversial rule, for both development and safety. On development, there are valid arguments on both sides. But from a safety standpoint, while it might seem a cautious approach (injured kids cannot return), in practice the opposite is more likely (kids who get knocked around hide the pain lest their day end with a substitution). At 13-16 years old, not something to encourage. I have known too many "tough" soccer players who were always playing through the pain, particularly on the girls side, whose bodies start breaking down between ages 17-21.