Yes, FORMAL is a Scrabble word!

Letter values in Scrabble US: F 4, O 1, R 1, M 3, A 1, L 1.

Is it a valid word?

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

being in accord with established forms and conventions and requirements (as e.g. of formal dress)
characteristic of or befitting a person in authority
(of spoken and written language) adhering to traditional standards of correctness and without casual, contracted, and colloquial forms
represented in simplified or symbolic form
logically deductive
refined or imposing in manner or appearance; befitting a royal court

Also: dinner dress, dinner gown, evening gown.