There is a spelling rule that C is silent in SC, with some exceptions. However there are so many exceptions that it is not much used as a rule. So I will explain why.
Most words where SC makes /sk/ are from (Norman) French.1 For example: scar (escare), scorn (escarnir), scour (escurer), screw (escroue), script (escrit)... Usually this combination is found at the beginning of a word. Although SC can be found in the middle of words from French. For example: escape (escaper), biscuit (bescuit) and obscure (obscur) or at the end as in mollusc (mollusque). A few are from Old English, ex: score, scab, or Old Norse, ex: scare, scrape.
Where SC makes /s/ the word usually comes from Latin.2 For example: ascend (ascendere), abscess (abscessus), discipline (disciplina) and rescind (rescindere). Usually this pattern is found in the middle of a word. Although it can, less commonly, be found at the beginning of the word. For example: sceptre / scepter3 (sceptrum), scene (scena4, scaena5), scissors (cisoria), science (scientia)...
Even when a word comes into English from Latin, a form of the word is (often) also found in French. The French language is descended from Latin and the language of the post-Roman Frankish invaders.6 This relationship makes determining the original source for an English word difficult. However in general it appears that where SC makes a /sk/, the word is often originally of northern European descent. Where SC makes /s/ the word is more likely to have come into English directly from Latin.
As a general guideline, SC is pronounced /s/ in words from Latin, otherwise it is /sk/. Since most people do not usually know a word's origin, a very rough guideline is the SC in the middle of a word is more often pronounced /s/, otherwise it is /sk/. Keep in mind that this is a general pattern and is not a rule. There are many exceptions.
C can also be pronounced with a ch-sound, a sh-sound or even be silent; these situations will be covered next week. Double-C will be covered in the following week.