Really need to know the rule limitations before any one can provide an appropriate answer. Be as specific as possible. Also helpful will be age & size of the youth and description of the course of fire (offhand, bench, kneeling, prone, "shooting sticks"?). Scope/open sights/peep sights - inlines? etc.
Ran into such things with my son starting out in an NRA YHEC shooting event, had to modify his rifle (.22) the day of the event. Not fun.