Yes, HIGHEST is a Scrabble word!

Letter values in Scrabble US: H 4, I 1, G 2, H 4, E 1, S 1, T 1.

Is it a valid word?

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

greater than normal in degree or intensity or amount
(literal meaning) being at or having a relatively great or specific elevation or upward extension (sometimes used in combinations like `knee-high')
standing above others in quality or position
used of sounds and voices; high in pitch or frequency
happy and excited and energetic
(used of the smell of meat) smelling spoiled or tainted
slightly and pleasantly intoxicated from alcohol or a drug (especially marijuana)

Also: high-pitched, in high spirits.