Just caught up on this thread.
I'm also a bit disappointed that this is a movie because much of the charm of HSI for me was the weekly anticipation, and discussions. HSI's week-to-week quality was so very variable that each week it was like opening up a box of chocolates, you really didn't know what you were going to get.
That, combined with the polarized nature of the viewer-base, made HSI discussion pretty fun (kind of like what Aquarion EVOL discussion is like now, lol).
That being said, a movie probably is the most natural extension of this property, because without a significant time-skip it's hard for me to see how they could get another two cours out of this cast. I mean, they did just about everything they could with Ohana and crew as high school students.
It'll also be interesting to see how P.A. Works does with a movie. If it's successful, this could cement P.A. Works shifting into being one of the "big boys" of the anime studio world.