Yes, POLITEST is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, L 1, I 1, T 1, E 1, S 1, T 1.

Is it a valid word?

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

showing regard for others in manners, speech, behavior, etc.
marked by refinement in taste and manners
not rude; marked by satisfactory (or especially minimal) adherence to social usages and sufficient but not noteworthy consideration for others