I'm going to respectfully disagree with Jim on this.
I don't believe the 40 second clock will ever come about in h.s. and I don't want it to, but I don't think the issue is the actual play clocks themselves. I've called games using NCAA rules where there is no visible play clock and the 40 second clock is kept by the back judge just as the 25 second clock is on Friday nights in lots of places. Officials who know the rules can do it just fine.
In reality, most plays have about that much time, anyway. I blow the ready for play (RFP) whistle between 12 and 15 seconds after the previous play ends so, adding 25 seconds, makes it a 37-40 second clock. The only difference is there is no whistle with 40 second clock.
Very few teams really need a play clock. I haven't had a delay of game flag in three years. Only got close once this season. Teams are better at lining up, getting the play called and snapping the ball. There is typically 10-20 seconds on the clock when the ball is snapped.
As for HR6 continually bitching about officials somehow hosing him off with slow whistles, I'll say this. It doesn't matter if the RFP is blown when they break the huddle or when the QB is calling signals.When a team breaks the huddle the ball will be snapped in 5-8 seconds in 99 percent of all cases. If the whistle has just sounded or it blew 10 seconds before makes NO DIFFERENCE. The pace of the offense is what it is. I do not believe any offense really cares when it's blown. Only a team with a lead would ever come to the line, figure out how much time they can bleed, then stand there and wait to snap the ball. As a caveat, if a team is running it's QB all the way to the sideline to talk after each play and get a play, and this is taking too long, every referee I know will hurry that up by NOT WAITING quite as long to blow the RFP.