Yes, MEDEVAC is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, D 2, E 1, V 4, A 1, C 3.

Is it a valid word?

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

the evacuation of persons (usually by air transportation) to a place where they can receive medical care

Also: medical evacuation.