I would be very concerned if I saw
a. a coach teaching bridging a sealing off to Juniors
b. a referee allowing it to happen in a Junior game
This practice is illegal, not only because it prevents a fair contest for the ball, but it is dangerous as exposes the "bridger's" neck and shoulders to significant impact as opponents arrive at the breakdown. That might be sustainable at the elite levels of the game, but at the community level, especially juniors, where it need to be stood-on immediately.
There was another thread on this some time ago, and it had this "coaching" video in it;
http://www.youtube.com/watch?v=jtpPV4Qdb0k
This is the current USA National Women's coach teaching Community level players how to
cheat. While she does show how the bridger can protect themselves from neck injury while they are cheating, this requires a degree of timing and co-ordination that is not readily apparent at the community level. They only have to get the timing slightly wrong and its broken neck time. There were at least a couple of junior girls at this coaching session. Not good.
The crack about "keeping it legal" made me smile. There is no legal way to bridge. Its cheating whichever way you slice it.. She seems to think that its only bridging if the player puts their hands on the ground or lays on the player in the ground. She's wrong. If the player puts their hand in the player on the ground, they are not standing in their feet unsupported... ping.