schedule

Donald Marsh wrote

I've been trying to find the origin for the difference in the pronunciation of the word 'schedule' (sked- and shed-) and thought you might be able to help. If you could give me a brief, layman's explanation as to why British English pronounces the word "shed-ule" and American English pronounces it "sked-ule" I would be deeply grateful. Which pronunciation is older, and how did the newer pronunciation become acceptable and take hold?

The basic answer is surely that we don’t know. As with most differences in pronunciation, we really cannot tell why one form has become prevalent in one geographical area and another in another.
I replied

It's often difficult or impossible to say WHY a particular pronunciation exists — ultimately it's a matter of fashion, like clothes.
For 'schedule', one mystery is that this word used to be pronounced with plain s-. That's the only form given by Walker (1791). (See the OED.) But it is never heard today.

The earliest spelling for this word given in the OED is cedule, from Old French and attested from the 14th to the 16th century, when the modern spelling first makes its appearance. (The OED continues, “mod.F. cédule”, though I can find no such word in my big Collins/Robert dictionary of French.) The pronunciation with /s-/ appears to have persisted unchanged. In BrE, the orthoepist Smart (1836) is the first to recommend the pronunciation with /ʃ-/. I don’t know what early Americans did.
So the question comes down to why /ˈsedjuːl/, the pronunciation used until the end of the 18th century, ultimately gave way to /ˈʃedjuːl/ in BrE but to /ˈskedʒ(u)əl/ in AmE.
Actually, as readers of LPD will know, the picture is actually more complicated than that. By no means all speakers of BrE prefer /ʃed-/. In my sample, in fact, 65% of BrE respondents born in 1974 or later preferred the /sked-/ form, which seems set to gradually displace /ʃed-/. This may have something to do with the fact that in AmE the word has a wider meaning than traditionally in BrE, since it covers BrE (railway, airline, bus) timetable as well as the meaning we share of a “tabular or classified statement” or “blank form”. Now we are borrowing this additional AmE timetable meaning (e.g. departing on schedule, scheduled arrival time), often along with its pronunciation.
I suppose that part of the answer to Mr Marsh’s question is that both /sk-/ and /ʃ-/ are spelling pronunciations. The model for the first would be in words such as school, scheme, schooner, Schofield; for the second, schnapps, schnozzle, meerschaum, and German names such as Schiller, Schubert, Schweppes, Porsche. Since the first group may be felt as more “native”, it is not surprising if it attracts items from the second. The same thing has happened with schism, traditionally /ˈsɪzəm/ but now usually /ˈskɪzəm/.
As for scheme, /k/ is the usual reflex of Greek chi, which is what σχῆμα schēma had.