Should High School rule be changed to allow both teams to sub on throw-in when team with throw subs . .. same as NCAA rule ?

There's no question this should be changed. High school is all about participation. The quicker we can get the substitutions in the game the more playing time everyone will have.

I agree; I am not certain what the college ruling is, but in a league I once directed [non-competitive, for a city recreation service] the best rule we found over the years was to allow the "non-throwing team" to sub if, and only if, the throwing team substituted; this allowed the possessing team to maintain momentum if they chose, but once the game was "interrupted" for subs, we let them all on

sub by both teams?
 

By rule in HS the team has to notify the referee (or nearest official) prior to the ball going out of play that they want to sub. Allowing both teams to substitute would only be feasible if both had previously notified the officials. The NFHS put the notification to speed up substitutions and to eliminate delays in the game. We need to remember participation includes keeping the game flowing for the enjoyment of the players on the field.