In practice most umpires will let belly rings and tongue piercings slide.
Nobody wants to start inspecting young ladies that closely.
Umpires aren't expected to do a full-body search looking for jewelry. No, we are not going to ask players to lift their shirts or open their mouths for inspection. It isn't really a matter of purposely "letting it slide". If it isn't visible, we can't specifically address it.
With respect to high school ball, most of the players should already be aware that they aren't supposed to wear jewelry. The coaches should be aware, too, as they are responsible to ensure that their players are legally and properly equipped. They are told this in preseason meetings and asked to confirm it to the plate umpire before every game.
So, if some kid gets her belly ring ripped out or cracks her teeth on a tongue ring, the plaintiffs attorney can go after the coach and the school board, because the umpire won't be responsible.