Not to play monkey in the middle, but the rule you quoted says "LEAVE the dugout DURING live ball". It doesn't say be out of the dugout...so while not in a live ball situation, say between innings, can the kid go sit/stand behind the backstop and stay there until a subsequent dead ball?
I don't want to speculate, but from previous experience, there are only 2 reasons to stand behind the backstop, try and get a closer to "batters view" of the opposing pitcher (judging their curveball, speed, etc), or try to find a "tell" that indicates whether they are throwing fastball/curveball/changeup, etc.
I myself have never encountered the situation ThaVoice described, but I am imagining a coach who knows the rule book (haha, that is a funny one), who quotes "but he didnt LEAVE during a live ball". Does that make a difference?
First off, I have encountered many, many coaches who know the rules. We don't mock coaches on here. Besides it being unprofessional based on you being an active umpire, I don't let posters mock umpires on here, so we aren't mocking posters, coaches, players or umpires. If you want to do that, create your own thread and do it there.
Next, there is a reason why we don't play "monkey in the middle" here. It strays from the original question (one that has already been properly answered) and usually ends up with someone trying to make something stick to the wall, that simply won't stick.... as is the case here.
Below are two official interpretations from the NFHS that are related to players or other team personnel being out of the dugout during a live ball. Please note that one situation the reason is justified and one is not.
Nowhere does it mention when the personnel left the dugout.
SITUATION 1: The home team has an enclosed batting cage behind its dugout. During the game, batters will take batting practice before batting in the game. The visiting coach complains to the plate umpire and wants the plate umpire to either allow his team to hit before its at-bats, or not allow anyone to use the batting cage during the game.
RULING: The umpire has no jurisdiction over the use of equipment or facilities outside the confines of the field. This is a matter that must be settled between the coaches or their league. However, if batting practice is taken during the game and the sound of the metal bats becomes distracting, the umpire can prohibit further use of the batting cage. (10-2-3; Approved Ruling 1990)
SITUATION 7: An assistant coach in the uniform of his team, is sitting in the stands recording the game and using a radar gun on his pitcher.
RULING: While it is legal for a camera or radar gun to be placed in the stands, it may not be used by a coach, player, attendant or other bench personnel. The assistant coach must be in the dugout or a dead-ball area inside the confines of the field. (3-3-1i)
In the original scenario presented, the player had no business being where he was. Whether or not he left the dugout when the ball was live in this situation is irrelevant.
Nobody can argue that the player had a valid reason to be where he was. Leaving the dugout to get to where he was when the ball was dead does not make his actions legal.