Welcome to rugbyrefs.com Kevin
I agree with Browner (wonders will never cease)
While it is technically legal to pull an opponent out of a ruck it is not something I would encourage players under the age of around 17 to do. There is a much wider range of relative physical strengths among juniors in the same age grades, We don't want some big strapping lad pulling a skinny kid out by his arm. This can cause serious injury if not done properly
Also you might be thinking of what is commonly termed a "saddle roll" or a "'gator roll". This is a technique whereby an arriving player, wishing to clean out an opponent at a tackle, will grasp the opponent around the chest (from above and behind) and roll him to the side and to ground to expose the ball. Players doing this must be careful not to grasp the player around the neck (Dangerous Play) or to attempt this technique once a ruck has been formed as it could collapse the ruck.
Again, this is not something I would encourage juniors to do due to the disparity in physical strengths encountered within age grades.
In addition to what Browner said about the iRB laws, you might also want to check whether your National Union has anything in their Junior guidelines. You local Referees Association/Society should be able to help you with this. Certain things are specifically outlawed by some unions (beyond the limitations already imposed in the iRB's U19 Variations). For example some unions prohibit line-out lifting and squeeze ball at certain levels.
NOTE: It is specifically prohibited to pull a player out of a maul - Law 17.3 (a)
