If a S is followed by a T or a P the S is pronounced like a Sch.
Examples:
Sturm -> pronounced Schturm
Spiel -> pronounced Schpiel.
If the S is followed by any other letter it remains a S.
Isch instead of Ich is what a lot of foreigners say who don't have German as their motherlanguage. I'm not aware of a regional German dialect that would pronounce ich like isch (although I think people who speak Swabian might do that....but I'm not sure...
).