Yes, PRINCIPALS is a Scrabble word!

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

Is it a valid word?

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

the original amount of a debt on which interest is calculated
the educator who has executive authority for a school
an actor who plays a principal role
capital as contrasted with the income derived from it
(criminal law) any person involved in a criminal offense, regardless of whether the person profits from such involvement
the major party to a financial transaction at a stock exchange; buys and sells for his own account

Also: school principal, head teacher, principal sum.