From today. Under 18 match. After a yellow card to the tight head, black (under pressure on their 5m line) told me that they would have to go to uncontested scrums as they had no other front row player trained. I doubted that as I saw plenty of them at the pre match brief. I called over to the bench and lo and behold a replacement came on and a flanker went off. But was this correct?
Yes. They have to take a player off so that the YC FR player can be replaced. The team can remove any player, except of course, they cannot remove another FR player.
I think in U18, if the YC scrum is down a player, the other team have to take a scrum player off too so its 7 v 7, and IIRC both teams have to pack a 3-4 scrum, i.e., no-one in the No. 8 position
[LAWS]LAW 3.5

After a front row player leaves the field of play or is sent off, or during the time for which a
front row player is temporarily suspended, the referee, upon awarding the next scrum, will
ask the player’s captain whether or not the team has another player on the playing area
who is suitably trained and experienced to play in the front row. If not, the captain chooses
another player on the playing area who then must leave the playing area to be replaced by a
suitably trained and experienced front row player from the team’s replacements. The captain
may do this when the scrum is ordered or after another player has been tried in the front
row.
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ETA: Yep, here are the relevant Laws
U19 VARIATIONS - LAW 20.1 FORMING A SCRUM
(f) In an 8 person scrum the formation must be 3-4-1, with the single player (normally the
Number 8) shoving on the 2 locks. The locks must pack with their heads on either side of
the hooker.
Exception: A team must have fewer than eight players in its scrum when the team cannot
field eight suitably trained players in its scrum due to either the team not fielding a
complete team, or a forward player being sent off or temporarily suspended for foul play, or
a forward player leaving the field because of injury.
Even allowing for this exception, each team must always have at least five players in a
scrum.
If a team is incomplete and it cannot field eight suitably trained players in its scrum, the
scrum formation must be as follows:
If a team is without one forward player, then both teams must use a 3-4 formation (i.e. no
No.8).[/LAWS]