Yes, SECESSION is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, C 3, E 1, S 1, S 1, I 1, O 1, N 1.

Is it a valid word?

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

an Austrian school of art and architecture parallel to the French art nouveau in the 1890s
the withdrawal of eleven southern states from the Union in 1860 which precipitated the American Civil War
formal separation from an alliance or federation

Not playable in Scrabble US: SEZESSION.