Most HS umpires that I know are pretty good at calling the curve ball. Most call it for where it crosses the plate. The ones who call it based on where the catcher secures it will give you the high curve and not the low curve. The ones who call it based on the plate will give you the low curve and deny the high curve. When a pitch is borderline and has a lot of late break at the top of the zone, the ball can trace around the zone without actually crossing it while still seeming like a strike from the dugout as it's crossing the batter who is actually behind the plate and not across from it.
The major issue when people think an umpire is tight is that they are watching from the side and seeing where these borderline breaking pitches cross the hitter and not where they cross the plate.
Most batters are at the severe back of the batter's box--well behind the plate.
To call the zone the umpire will imagine a lane over the plate the width of the plate plus 3-4 inches to each side . Then the umpire visualizes the batter's stance as he prepares to swing at the release of the pitch--not when he's just standing there watching the first part of the delivery. This visual provides the upper and lower limits and completes the mental cube.
At the youth level, umpires are not used to calling breaking pitches and give up on them before they break--thinking it was a change up gone astray or a fastball that has no chance. Once the concentration is broken, they are really just guessing. The more they see curve balls, the better they get...in theory anyway.
Does that help?