Yes, TETHERBALLS is a Scrabble word!

Letter values in Scrabble US: T 1, E 1, T 1, H 4, E 1, R 1, B 3, A 1, L 1, L 1, S 1.

Is it a valid word?

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

a game with two players who use rackets to strike a ball that is tethered to the top of a pole; the object is to wrap the string around the pole