I suspect you will find that it is nothing until something happens. I am not sure of HS rules here but the runner probably needs to make an attempt to reach the plate by sliding or going around the catcher. At that point the umpire has something to call - if the runner checks up as described then the umpire calls him out because there was nothing to call.
Think of it like this - runner on 2nd ball hit in 3B hole - runner coming down from 2nd checks up to avoid contact with 3B who was playing even with the bag and dives into the hole. Runner then tries to score and is thrown out by 2 steps at the plate. Nothing for umpire to call. If on the other hand the ball is past the 3B and he makes contact with the runner - then you get the obstruction call.
One thing to be careful of is that it does not guarantee being safe at home on this play. If the runner in the umpire judgment would have been out at home without the obstruction he will still be out. This happened some years ago in the playoffs when a runner didn't go hard all the way and got thrown out by 25 feet.