We-ee-ee-ll it is possible to produce a technical counter-argument, which is doubtless where these youths were coming from. I expect they attend a very good school, as your bog-standard sink Comprehensive tends not to enter the Vase.
a) From the moment the scrum forms to the moment the scrum starts, all FR players must be in a position to shove. [20.2(a)]
b) During that time, the LHP must not cross his feet [20.2(b)]
c) Scrum begins when the ball leaves the SH's hands [20.7(a)]
d) The LHP is not allowed to hook for the ball until after it bounces [20.8(b)]
e) The ball must bounce beyond the LHP's inside shoulder [20.6(d)]
Looking at all that, we might interpret d) to mean that the LHP's hooking foot cannot leave the ground until after the bounce. The actual wording is:
[LAWS](b) Striking after the throw-in. Once the ball touches the ground in the tunnel, any front row player may use either foot to try to win possession of the ball.[/LAWS]
There is a clear implication that the LHP putting his foot up before the bounce is an offence. That leaves us with the oppo hooker's foot much closer to the ball than the LHP (unless he's hooking with his inside foot); if the LHP gets there first with the outside foot, we may presume either that the oppo hooker is unutterably useless, or that the LHP had his foot up before the bounce.
So your youthful interlocutor may have had a point.