Yes, EARLIEST is a Scrabble word!

Letter values in Scrabble US: E 1, A 1, R 1, L 1, I 1, E 1, S 1, T 1.

Is it a valid word?

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

(comparative and superlative of `early') more early than; most early
at or near the beginning of a period of time or course of events or before the usual or expected time
being or occurring at an early stage of development
belonging to the distant past
very young
of an early stage in the development of a language or literature
expected in the near future