Yes, NINJAS is a Scrabble word!

Letter values in Scrabble US: N 1, I 1, N 1, J 8, A 1, S 1.

Is it a valid word?

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

a member of the ninja who were trained in martial arts and hired for espionage or sabotage or assassinations; a person skilled in ninjutsu
a class of 14th century Japanese who were trained in martial arts and were hired for espionage and assassinations