Yes, PRINCIPLES is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, I 1, N 1, C 3, I 1, P 3, L 1, E 1, S 1.

Is it a valid word?

PRINCIPLES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct
a rule or standard especially of good behavior
a basic truth or law or assumption
a rule or law concerning a natural phenomenon or the function of a complex system
rule of personal conduct
(law) an explanation of the fundamental reasons (especially an explanation of the working of some device in terms of laws of nature)