SOLVED: B-movie from early 1980s (?), title ?

Hi folks,
I remember having seen an old B-class horror movie (which was part of a multi-disc set I just can't find anymore...) - maybe someone can identify it:
A school bus drives into a strange cloud, on a lonesome road. Later, when the bus hasn't arrived, a search party finds it, empty... all the kids are gone!
When some of them return, days later, they seem to be unharmed (but that's an illusion!) - somehow the kids have changed: It's shown several times in the movie that when the kid hugs an adult, the latter one burns to death (?) or, maybe I should rather call it, he/she "melts away"...
Very strange. And with full of plot holes /missing explanations whatsoever... AND I can't remember how it all ended... But I really would like to search for another copy, should the box set never re-surface from my chaotic dvd treasure vaults ;-D
Still a quite funny and (unintentionally, of course ;-) crappy B-movie ("so good it's bad", as it's called)
Anyone knows the title?
I think the title had the word "rain" in it (I don't mean Black Rain, 1989) although the events weren't started by poisoned rain but rather by a mysterious fog, into which the bus drove. But it hadn't to do anything with rain. Ah, these are the real "masterworks" which don't even lose 5 minute's time into the movie before showing the first inconsistency! And, that wasn't the last one...
Kind regards
Andreas (123all4me)