Yes, HIGHS is a Scrabble word!

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

Is it a valid word?

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

a lofty level or position or degree
an air mass of higher than normal pressure
a state of sustained elation
a state of altered consciousness induced by alcohol or narcotics
a high place
a public secondary school usually including grades 9 through 12
a forward gear with a gear ratio that gives the greatest vehicle velocity for a given engine speed

Also: senior high school, senior high, high school, high gear.

Not playable in Scrabble US: HIGHSCHOOL.