Yes, ASSIZES is a Scrabble word!

Letter values in Scrabble US: A 1, S 1, S 1, I 1, Z 10, E 1, S 1.

Is it a valid word?

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

the county courts of England (replaced in 1971 by Crown courts)
the regulation of weights and measures of articles offered for sale
an ancient writ issued by a court of assize to the sheriff for the recovery of property

Also: court of assize, court of assize and nisi prius.