It’s all a muddle, because English applies a hodgepodge of pronunciation and spelling rules from various Germanic and Latinate languages to various Latinate and Germanic words. A very small note in the trusty Oxford English Dictionary says “The current spelling in sc- is attested from the mid 16th cent., and appears to reflect analogical remodelling after e.g. scene n., sceptre n., etc.” The word in Middle French from which it derives (the verb “sentir” – to perceive by odor, from Latin sentīre”) had no c. In this case, then, the c appears to be silent, a kind of quiet interloper from similar words borrowed from French.