Are there any rules for the pronunciation of Ch words. In words like, for example, chess, chemistry, school, etc. I want to know when should we pronounce 'ch' as "K' and when this has to be pronounced as "tʃ" as in chess.
The pronunciation of ⟨ch⟩ as /k/ is generally found in words borrowed from Greek (where the ⟨ch⟩ stands for the Greek letter chi). See Wikipedia: English words of Greek origin:
Ch is pronounced like k rather than as in "church": e.g., character, chaos.
It's annoyingly hard to find a non-Wikipedia reference, but this borders on common knowledge.
Loanwords from a few other languages have ⟨ch⟩ pronounced as /k/— for example, dachshund (a type of dog; from German) or loch ("lake"; from Scots). The sound in the original language was an /x/, not a /k/. Certain German names (or names from other languages transliterated in a German-ish way) are pronounced similarly: Bach (German composer), Friedrich (common German first name), Rachmaninoff (Russian composer; the "scientific" transliteration of his name would be Rakhmaninov). There are also a few from Italian, where ⟨ch⟩ does stand for /k/: bruschetta (a type of snack with tomatoes on toasted bread; often mispronounced!), Chianti (a type of wine), Pinocchio (a character from a famous children's book).
You'll also find this question relevant: How do I know when a word with "ch" is pronounced hard or softly?