The U19 Variation for intentionally keeping the ball in the scrum is a FK:
[LAWS]20.9(k) Ball must be released from scrum. A player must not intentionally keep the ball in the scrum once the player’s team has heeled the ball and controls it at the base of the scrum.
Sanction: Free Kick[/LAWS]
Given the absence of a stated sanction, you could (a) re-set same team put in (based on new wheel re-start); (b) re-set, other team put in (based on inability of team to use the ball at their No 8's feet); or (c) FK, if there appears to be intention, or just bloody-mindness. It'll probably depend on hwo the game has been going ... and if any team refuses to / is unable to respond. Probability?