Yes, STRATUM is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, R 1, A 1, T 1, U 1, M 3.

Is it a valid word?

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

one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)
people having the same social, economic, or educational status
an abstract place usually conceived as having depth

Also: social class, socio-economic class.