Yes, ES is a Scrabble word!

Letter values in Scrabble US: E 1, S 1.

Is it a valid word?

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

a radioactive transuranic element produced by bombarding plutonium with neutrons
a fat-soluble vitamin that is essential for normal reproduction; an important antioxidant that neutralizes free radicals in the body
a radioactive transuranic element produced by bombarding plutonium with neutrons
the cardinal compass point that is at 90 degrees
the base of the natural system of logarithms; approximately equal to 2.718282...
the 5th letter of the Roman alphabet

Also: e, atomic number 99, vitamin e, due east.