I went to a national school some years ago, in Philly, and thought it was well worth the time & expense. Not from what I learned at the school - that seemed, as others have noted, to be aimed at too low a level. Heck, I even found a volunteer catcher who had a hard time catching - I think he had a pitch bounce off of his knees
I want to highlight that the hospitality room was where the real benefit is. Where else can you have a 1-on-1 conversation with a NUS member -to say nothing of the regional or state uic's that are present?! I found a real benefit to this. And I met a number of other umpires from my region and others that confirmed some impressions and corrected a few misconceptions.
If you get a chance to go to one of these, go.
As for the advanced national school, maybe someday. Still too much going on now to take the time to attend.