There is no productive phonetic rule in High German which merges /s/ + /ç/ → /ʃ/ or /s/ + /x/ → /ʃ/, so the pronunciation of the trigraph ‘sch’ always [*] depends on the pronunciation of the components – and when there are no components, it should be a /ʃ/.
Historically, though, there was a merger for at least /s/ + /k/ → /ʃ/ or /s/ + /x/ → /ʃ/ which is evidenced by Old High German scrîban (High German schreiben, to write, cf. Swedish skriva). (See also Wikipedia: http://de.wikipedia.org/wiki/Sch_(Trigraph))
[*]: As has been mentioned, there are exceptions for loanwords, which – possibly under the influence of their spelling – have changed their pronunciation to /ʃ/; and on the other hand, there are folk etymological re-interpretations for the pronunciation of the ‘sch’ in the Röschenhof. Lacking more examples, I don’t think this is a productive rule.