Posted: Apr 16, 2012 9:49 am
The sound changes given above are only a small set - sometimes, even the opposite change occurs, and these are far, far from all known sound changes - and there's probably way more possible changes around. (Sometimes, sounds come from nowhere as well - Spanish for instance, has inserted vowels ex nihilo into words that begin with certain clusters, c.f. Italian scuola, Spanish escuela.)
Concerning this specific sound change (which we also find in French, but in general they also drop the "s", hence école), could this be caused by a people originally speaking another language (Gaulish for French, Celtiberian for Spanish - both Celtic languages) adopting another language (vulgar Latin in both cases) and imposing some of the speech patterns of their original language on it?
(I once read (old) French being described as Latin mangled by Gaulish throats).